How to modify image caption in posts?

I am trying to modify the caption of images in posts by filtering img_caption_shortcode, and then displaying the content using the_content(). I want to add bylines/credit on images, so I have created two fields in ACF that should be added inside the figcaption on all my posts. Basically I want to do what is description in the blog.

So from this:

figcaption
So this is a caption that describes the image.
/figcaption

To this:

figcaption class=post-thumbnail-caption
So this is a caption that describes the image.
span class=wp-block-image-bylinePhoto: Gregor Gregson/span
/figcaption

I have tried just doing it as simple as possible, but I can not get it to work. Nothing happans/changes with the following code in my functions.php (I did first try the whole code from the example in the blog linked above).

add_filter( 'img_caption_shortcode', 'my_img_caption_shortcode', 10, 3 );
 
function my_img_caption_shortcode( $output, $attr, $content ) {
    $output = 'test';
    return $output;
 
}

Any ideas on what I could be doing wrong?