Gutenberg Block manipulation: Undo parse_blocks() with serialize_blocks() results in unicode issues

I would like to manipulate Gutenberg Blocks with PHP. Therefore I need to parse post_content into an array and the same way back after manipulation. But I don't get that far, because already the parsing back and forth doesn't work.

function change_post_data_before_save( $data, $postarr ) {
    $post_data = $data['post_content']; //post_content seems to be json encoded 
    //stripping slashes otherwise $blocks[0]['attrs'] would be NULL
    $post_data = str_replace('\\', '', $post_data);
    //trying to replace double slash in unicodes but it seems not to work
    $post_data = str_replace('\\\u', '\u', $post_data);
    $blocks = parse_blocks($post_data); //there are still some issues with schema and other array parts

    //manipulation of $blocks if needed

    $post_content = serialize_blocks( $blocks ); //seems to work correct
    $data['post_content'] = $post_content;

    return $data;
}
add_filter( 'wp_insert_post_data', 'change_post_data_before_save', 10, 2 );

Any ideas how to json decode $data['post_data'] without destroying parts in $blocks[0]['attrs'] which need to be json encoded? I tried replacement but it seems not to work for unicodes and serialize_blocks() destroyes it in the end.

Example Code:

//expected result - saved in database without wp_insert_post_data (see above)
!-- wp:uagb/faq {block_id:f1ad85bc,enableSchemaSupport:true,schema:{\u0022@context\u0022:\u0022https://schema.org\u0022,\u0022@type\u0022:\u0022FAQPage\u0022,\u0022@id\u0022:\u0022http://localhost/specials/gutenberg/\u0022,\u0022mainEntity\u0022:[{\u0022@type\u0022:\u0022Question\u0022,\u0022name\u0022:\u0022Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\u0022,\u0022acceptedAnswer\u0022:{\u0022@type\u0022:\u0022Answer\u0022,\u0022text\u0022:\u0022This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\u0022}}]}} --
div class=wp-block-uagb-faq uagb-faq__outer-wrap uagb-block-f1ad85bc uagb-faq-icon-row uagb-faq-layout-accordion uagb-faq-expand-first-false uagb-faq-inactive-other-true uagb-faq-equal-height data-faqtoggle=true role=tablistscript type=application/ld+json{@context:https://schema.org,@type:FAQPage,@id:http://localhost/specials/gutenberg/,mainEntity:[{@type:Question,name:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),acceptedAnswer:{@type:Answer,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)}}]}/scriptdiv class=uagb-faq__wrap uagb-buttons-layout-wrap!-- wp:uagb/faq-child {block_id:61cac9b0,question:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),answer:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)} --
div class=wp-block-uagb-faq-child uagb-faq-child__outer-wrap uagb-block-61cac9b0div class=uagb-faq-child__wrapperdiv class=uagb-faq-item role=tab tabindex=0div class=uagb-faq-questions-button uagb-faq-questionsspan class=uagb-icon uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H272V64c0-17.67-14.33-32-32-32h-32c-17.67 0-32 14.33-32 32v144H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h144v144c0 17.67 14.33 32 32 32h32c17.67 0 32-14.33 32-32V304h144c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-icon-active uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h384c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-questionHeadline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/divdiv class=uagb-faq-contentspanpThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/p/span/div/div/div/div
!-- /wp:uagb/faq-child --/div/div
!-- /wp:uagb/faq --

!-- wp:kadence/iconlist {items:[{icon:fe_alertCircle,link:,target:_self,size:20,width:2,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),color:,background:,border:,borderRadius:0,padding:5,borderWidth:1,style:default}],uniqueID:_a79c18-e7} --
div class=wp-block-kadence-iconlist kt-svg-icon-list-items kt-svg-icon-list-items_a79c18-e7 kt-svg-icon-list-columns-1 alignnoneul class=kt-svg-icon-listli class=kt-svg-icon-list-style-default kt-svg-icon-list-item-wrap kt-svg-icon-list-item-0div style=display:inline-flex;justify-content:center;align-items:center class=kt-svg-icon-list-single kt-svg-icon-list-single-fe_alertCirclesvg style=display:inline-block;vertical-align:middle viewbox=0 0 24 24 height=20 width=20 fill=none stroke=currentColor xmlns=http://www.w3.org/2000/svg stroke-width=2 stroke-linecap=round stroke-linejoin=roundcircle cx=12 cy=12 r=10/circleline x1=12 y1=8 x2=12 y2=12/lineline x1=12 y1=16 x2=12 y2=16/line/svg/divspan class=kt-svg-icon-list-textThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/li/ul/div
!-- /wp:kadence/iconlist --

Debugging:

//result of $data['post_content']
!-- wp:uagb/faq {\\block_id\\:\\f1ad85bc\\,\\enableSchemaSupport\\:true,\\schema\\:\\{\\\\u0022@context\\\\u0022:\\\\u0022https://schema.org\\\\u0022,\\\\u0022@type\\\\u0022:\\\\u0022FAQPage\\\\u0022,\\\\u0022@id\\\\u0022:\\\\u0022http://localhost/specials/gutenberg/\\\\u0022,\\\\u0022mainEntity\\\\u0022:[{\\\\u0022@type\\\\u0022:\\\\u0022Question\\\\u0022,\\\\u0022name\\\\u0022:\\\\u0022Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\\\u0022,\\\\u0022acceptedAnswer\\\\u0022:{\\\\u0022@type\\\\u0022:\\\\u0022Answer\\\\u0022,\\\\u0022text\\\\u0022:\\\\u0022This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\\\u0022}}]}\\} --
div class=\\wp-block-uagb-faq uagb-faq__outer-wrap uagb-block-f1ad85bc uagb-faq-icon-row uagb-faq-layout-accordion uagb-faq-expand-first-false uagb-faq-inactive-other-true uagb-faq-equal-height\\ data-faqtoggle=\\true\\ role=\\tablist\\script type=\\application/ld+json\\{\\@context\\:\\https://schema.org\\,\\@type\\:\\FAQPage\\,\\@id\\:\\http://localhost/specials/gutenberg/\\,\\mainEntity\\:[{\\@type\\:\\Question\\,\\name\\:\\Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\,\\acceptedAnswer\\:{\\@type\\:\\Answer\\,\\text\\:\\This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\}}]}/scriptdiv class=\\uagb-faq__wrap uagb-buttons-layout-wrap\\!-- wp:uagb/faq-child {\\block_id\\:\\61cac9b0\\,\\question\\:\\Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\,\\answer\\:\\This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\} --
div class=\\wp-block-uagb-faq-child uagb-faq-child__outer-wrap uagb-block-61cac9b0\\div class=\\uagb-faq-child__wrapper\\div class=\\uagb-faq-item\\ role=\\tab\\ tabindex=\\0\\div class=\\uagb-faq-questions-button uagb-faq-questions\\span class=\\uagb-icon uagb-faq-icon-wrap\\svg xmlns=\\http://www.w3.org/2000/svg\\ viewbox=\\0 0 448 512\\path d=\\M416 208H272V64c0-17.67-14.33-32-32-32h-32c-17.67 0-32 14.33-32 32v144H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h144v144c0 17.67 14.33 32 32 32h32c17.67 0 32-14.33 32-32V304h144c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z\\/path/svg/spanspan class=\\uagb-icon-active uagb-faq-icon-wrap\\svg xmlns=\\http://www.w3.org/2000/svg\\ viewbox=\\0 0 448 512\\path d=\\M416 208H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h384c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z\\/path/svg/spanspan class=\\uagb-question\\Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/divdiv class=\\uagb-faq-content\\spanpThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/p/span/div/div/div/div
!-- /wp:uagb/faq-child --/div/div
!-- /wp:uagb/faq --

!-- wp:kadence/iconlist {\\items\\:[{\\icon\\:\\fe_alertCircle\\,\\link\\:\\\\,\\target\\:\\_self\\,\\size\\:20,\\width\\:2,\\text\\:\\This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\,\\color\\:\\\\,\\background\\:\\\\,\\border\\:\\\\,\\borderRadius\\:0,\\padding\\:5,\\borderWidth\\:1,\\style\\:\\default\\}],\\uniqueID\\:\\_a79c18-e7\\} --
div class=\\wp-block-kadence-iconlist kt-svg-icon-list-items kt-svg-icon-list-items_a79c18-e7 kt-svg-icon-list-columns-1 alignnone\\ul class=\\kt-svg-icon-list\\li class=\\kt-svg-icon-list-style-default kt-svg-icon-list-item-wrap kt-svg-icon-list-item-0\\div style=\\display:inline-flex;justify-content:center;align-items:center\\ class=\\kt-svg-icon-list-single kt-svg-icon-list-single-fe_alertCircle\\svg style=\\display:inline-block;vertical-align:middle\\ viewbox=\\0 0 24 24\\ height=\\20\\ width=\\20\\ fill=\\none\\ stroke=\\currentColor\\ xmlns=\\http://www.w3.org/2000/svg\\ stroke-width=\\2\\ stroke-linecap=\\round\\ stroke-linejoin=\\round\\circle cx=\\12\\ cy=\\12\\ r=\\10\\/circleline x1=\\12\\ y1=\\8\\ x2=\\12\\ y2=\\12\\/lineline x1=\\12\\ y1=\\16\\ x2=\\12\\ y2=\\16\\/line/svg/divspan class=\\kt-svg-icon-list-text\\This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/li/ul/div
!-- /wp:kadence/iconlist --

//result of $post_data
!-- wp:uagb/faq {block_id:f1ad85bc,enableSchemaSupport:true,schema:{\\u0022@context\\u0022:\\u0022https://schema.org\\u0022,\\u0022@type\\u0022:\\u0022FAQPage\\u0022,\\u0022@id\\u0022:\\u0022http://localhost/specials/gutenberg/\\u0022,\\u0022mainEntity\\u0022:[{\\u0022@type\\u0022:\\u0022Question\\u0022,\\u0022name\\u0022:\\u0022Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\u0022,\\u0022acceptedAnswer\\u0022:{\\u0022@type\\u0022:\\u0022Answer\\u0022,\\u0022text\\u0022:\\u0022This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)\\u0022}}]}} --
div class=wp-block-uagb-faq uagb-faq__outer-wrap uagb-block-f1ad85bc uagb-faq-icon-row uagb-faq-layout-accordion uagb-faq-expand-first-false uagb-faq-inactive-other-true uagb-faq-equal-height data-faqtoggle=true role=tablistscript type=application/ld+json{@context:https://schema.org,@type:FAQPage,@id:http://localhost/specials/gutenberg/,mainEntity:[{@type:Question,name:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),acceptedAnswer:{@type:Answer,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)}}]}/scriptdiv class=uagb-faq__wrap uagb-buttons-layout-wrap!-- wp:uagb/faq-child {block_id:61cac9b0,question:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),answer:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)} --
div class=wp-block-uagb-faq-child uagb-faq-child__outer-wrap uagb-block-61cac9b0div class=uagb-faq-child__wrapperdiv class=uagb-faq-item role=tab tabindex=0div class=uagb-faq-questions-button uagb-faq-questionsspan class=uagb-icon uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H272V64c0-17.67-14.33-32-32-32h-32c-17.67 0-32 14.33-32 32v144H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h144v144c0 17.67 14.33 32 32 32h32c17.67 0 32-14.33 32-32V304h144c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-icon-active uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h384c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-questionHeadline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/divdiv class=uagb-faq-contentspanpThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/p/span/div/div/div/div
!-- /wp:uagb/faq-child --/div/div
!-- /wp:uagb/faq --

!-- wp:kadence/iconlist {items:[{icon:fe_alertCircle,link:,target:_self,size:20,width:2,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),color:,background:,border:,borderRadius:0,padding:5,borderWidth:1,style:default}],uniqueID:_a79c18-e7} --
div class=wp-block-kadence-iconlist kt-svg-icon-list-items kt-svg-icon-list-items_a79c18-e7 kt-svg-icon-list-columns-1 alignnoneul class=kt-svg-icon-listli class=kt-svg-icon-list-style-default kt-svg-icon-list-item-wrap kt-svg-icon-list-item-0div style=display:inline-flex;justify-content:center;align-items:center class=kt-svg-icon-list-single kt-svg-icon-list-single-fe_alertCirclesvg style=display:inline-block;vertical-align:middle viewbox=0 0 24 24 height=20 width=20 fill=none stroke=currentColor xmlns=http://www.w3.org/2000/svg stroke-width=2 stroke-linecap=round stroke-linejoin=roundcircle cx=12 cy=12 r=10/circleline x1=12 y1=8 x2=12 y2=12/lineline x1=12 y1=16 x2=12 y2=16/line/svg/divspan class=kt-svg-icon-list-textThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/li/ul/div
!-- /wp:kadence/iconlist --

//result of $blocks
array (
  0 = 
  array (
    'blockName' = 'uagb/faq',
    'attrs' = 
    array (
      'block_id' = 'f1ad85bc',
      'enableSchemaSupport' = true,
      'schema' = '{@context:https://schema.org,@type:FAQPage,@id:http://localhost/specials/gutenberg/,mainEntity:[{@type:Question,name:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),acceptedAnswer:{@type:Answer,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)}}]}',
    ),
    'innerBlocks' = 
    array (
      0 = 
      array (
        'blockName' = 'uagb/faq-child',
        'attrs' = 
        array (
          'block_id' = '61cac9b0',
          'question' = 'Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)',
          'answer' = 'This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)',
        ),
        'innerBlocks' = 
        array (
        ),
        'innerHTML' = '
div class=wp-block-uagb-faq-child uagb-faq-child__outer-wrap uagb-block-61cac9b0div class=uagb-faq-child__wrapperdiv class=uagb-faq-item role=tab tabindex=0div class=uagb-faq-questions-button uagb-faq-questionsspan class=uagb-icon uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H272V64c0-17.67-14.33-32-32-32h-32c-17.67 0-32 14.33-32 32v144H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h144v144c0 17.67 14.33 32 32 32h32c17.67 0 32-14.33 32-32V304h144c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-icon-active uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h384c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-questionHeadline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/divdiv class=uagb-faq-contentspanpThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/p/span/div/div/div/div
',
        'innerContent' = 
        array (
          0 = '
div class=wp-block-uagb-faq-child uagb-faq-child__outer-wrap uagb-block-61cac9b0div class=uagb-faq-child__wrapperdiv class=uagb-faq-item role=tab tabindex=0div class=uagb-faq-questions-button uagb-faq-questionsspan class=uagb-icon uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H272V64c0-17.67-14.33-32-32-32h-32c-17.67 0-32 14.33-32 32v144H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h144v144c0 17.67 14.33 32 32 32h32c17.67 0 32-14.33 32-32V304h144c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-icon-active uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h384c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-questionHeadline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/divdiv class=uagb-faq-contentspanpThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/p/span/div/div/div/div
',
        ),
      ),
    ),
    'innerHTML' = '
div class=wp-block-uagb-faq uagb-faq__outer-wrap uagb-block-f1ad85bc uagb-faq-icon-row uagb-faq-layout-accordion uagb-faq-expand-first-false uagb-faq-inactive-other-true uagb-faq-equal-height data-faqtoggle=true role=tablistscript type=application/ld+json{@context:https://schema.org,@type:FAQPage,@id:http://localhost/specials/gutenberg/,mainEntity:[{@type:Question,name:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),acceptedAnswer:{@type:Answer,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)}}]}/scriptdiv class=uagb-faq__wrap uagb-buttons-layout-wrap/div/div
',
    'innerContent' = 
    array (
      0 = '
div class=wp-block-uagb-faq uagb-faq__outer-wrap uagb-block-f1ad85bc uagb-faq-icon-row uagb-faq-layout-accordion uagb-faq-expand-first-false uagb-faq-inactive-other-true uagb-faq-equal-height data-faqtoggle=true role=tablistscript type=application/ld+json{@context:https://schema.org,@type:FAQPage,@id:http://localhost/specials/gutenberg/,mainEntity:[{@type:Question,name:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),acceptedAnswer:{@type:Answer,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)}}]}/scriptdiv class=uagb-faq__wrap uagb-buttons-layout-wrap',
      1 = NULL,
      2 = '/div/div
',
    ),
  ),
  1 = 
  array (
    'blockName' = NULL,
    'attrs' = 
    array (
    ),
    'innerBlocks' = 
    array (
    ),
    'innerHTML' = '

',
    'innerContent' = 
    array (
      0 = '

',
    ),
  ),
  2 = 
  array (
    'blockName' = 'kadence/iconlist',
    'attrs' = 
    array (
      'items' = 
      array (
        0 = 
        array (
          'icon' = 'fe_alertCircle',
          'link' = '',
          'target' = '_self',
          'size' = 20,
          'width' = 2,
          'text' = 'This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)',
          'color' = '',
          'background' = '',
          'border' = '',
          'borderRadius' = 0,
          'padding' = 5,
          'borderWidth' = 1,
          'style' = 'default',
        ),
      ),
      'uniqueID' = '_a79c18-e7',
    ),
    'innerBlocks' = 
    array (
    ),
    'innerHTML' = '
div class=wp-block-kadence-iconlist kt-svg-icon-list-items kt-svg-icon-list-items_a79c18-e7 kt-svg-icon-list-columns-1 alignnoneul class=kt-svg-icon-listli class=kt-svg-icon-list-style-default kt-svg-icon-list-item-wrap kt-svg-icon-list-item-0div style=display:inline-flex;justify-content:center;align-items:center class=kt-svg-icon-list-single kt-svg-icon-list-single-fe_alertCirclesvg style=display:inline-block;vertical-align:middle viewbox=0 0 24 24 height=20 width=20 fill=none stroke=currentColor xmlns=http://www.w3.org/2000/svg stroke-width=2 stroke-linecap=round stroke-linejoin=roundcircle cx=12 cy=12 r=10/circleline x1=12 y1=8 x2=12 y2=12/lineline x1=12 y1=16 x2=12 y2=16/line/svg/divspan class=kt-svg-icon-list-textThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/li/ul/div
',
    'innerContent' = 
    array (
      0 = '
div class=wp-block-kadence-iconlist kt-svg-icon-list-items kt-svg-icon-list-items_a79c18-e7 kt-svg-icon-list-columns-1 alignnoneul class=kt-svg-icon-listli class=kt-svg-icon-list-style-default kt-svg-icon-list-item-wrap kt-svg-icon-list-item-0div style=display:inline-flex;justify-content:center;align-items:center class=kt-svg-icon-list-single kt-svg-icon-list-single-fe_alertCirclesvg style=display:inline-block;vertical-align:middle viewbox=0 0 24 24 height=20 width=20 fill=none stroke=currentColor xmlns=http://www.w3.org/2000/svg stroke-width=2 stroke-linecap=round stroke-linejoin=roundcircle cx=12 cy=12 r=10/circleline x1=12 y1=8 x2=12 y2=12/lineline x1=12 y1=16 x2=12 y2=16/line/svg/divspan class=kt-svg-icon-list-textThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/li/ul/div
',
    ),
  ),
)

Final result after serialize_blocks():

//result of $post_content [can not be shown in Gutenberg UI This block contains unexpected or invalid content.]
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div class=wp-block-uagb-faq uagb-faq__outer-wrap uagb-block-f1ad85bc uagb-faq-icon-row uagb-faq-layout-accordion uagb-faq-expand-first-false uagb-faq-inactive-other-true uagb-faq-equal-height data-faqtoggle=true role=tablistscript type=application/ld+json{@context:https://schema.org,@type:FAQPage,@id:http://localhost/specials/gutenberg/,mainEntity:[{@type:Question,name:Headline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü),acceptedAnswer:{@type:Answer,text:This is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)}}]}/scriptdiv class=uagb-faq__wrap uagb-buttons-layout-wrap!-- wp:uagb/faq-child {block_id:61cac9b0,question:Headline with special characters: (\\u00b0C, \\u00e4, \\u00f6, \\u00fc, \\u00df, \\u00d6, \\u00c4, \\u00dc),answer:This is a test with special characters: (\\u00b0C, \\u00e4, \\u00f6, \\u00fc, \\u00df, \\u00d6, \\u00c4, \\u00dc)} --
div class=wp-block-uagb-faq-child uagb-faq-child__outer-wrap uagb-block-61cac9b0div class=uagb-faq-child__wrapperdiv class=uagb-faq-item role=tab tabindex=0div class=uagb-faq-questions-button uagb-faq-questionsspan class=uagb-icon uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H272V64c0-17.67-14.33-32-32-32h-32c-17.67 0-32 14.33-32 32v144H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h144v144c0 17.67 14.33 32 32 32h32c17.67 0 32-14.33 32-32V304h144c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-icon-active uagb-faq-icon-wrapsvg xmlns=http://www.w3.org/2000/svg viewbox=0 0 448 512path d=M416 208H32c-17.67 0-32 14.33-32 32v32c0 17.67 14.33 32 32 32h384c17.67 0 32-14.33 32-32v-32c0-17.67-14.33-32-32-32z/path/svg/spanspan class=uagb-questionHeadline with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/divdiv class=uagb-faq-contentspanpThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/p/span/div/div/div/div
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div class=wp-block-kadence-iconlist kt-svg-icon-list-items kt-svg-icon-list-items_a79c18-e7 kt-svg-icon-list-columns-1 alignnoneul class=kt-svg-icon-listli class=kt-svg-icon-list-style-default kt-svg-icon-list-item-wrap kt-svg-icon-list-item-0div style=display:inline-flex;justify-content:center;align-items:center class=kt-svg-icon-list-single kt-svg-icon-list-single-fe_alertCirclesvg style=display:inline-block;vertical-align:middle viewbox=0 0 24 24 height=20 width=20 fill=none stroke=currentColor xmlns=http://www.w3.org/2000/svg stroke-width=2 stroke-linecap=round stroke-linejoin=roundcircle cx=12 cy=12 r=10/circleline x1=12 y1=8 x2=12 y2=12/lineline x1=12 y1=16 x2=12 y2=16/line/svg/divspan class=kt-svg-icon-list-textThis is a test with special characters: (°C, ä, ö, ü, ß, Ö, Ä, Ü)/span/li/ul/div
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Related Topic: serialize_blocks breaking html tags in content