Why is Wordpress swapping double quotes (") to curly quotes (“) and how do I stop it from happening?

I have imported HTML into a Wordpress page, it contains some HTML as follows:

a class=fancybox href=bigger-image-path.jpg title=emThis is the image caption,/em by someone
 picture
  source media=(min-width: 768px) srcset=/image-path.jpg 1x, /image-path.jpg 2x/
  img class=image-no-border style=width:100%; height:auto src=/image-path.jpg alt=alt
 /picture
/a

When I look at this HTML once it is imported via phpmyadmin, the HTML all looks correct as it is above.

But when I look at the page source, the HTML has been changed somehow to the following:

a class=fancybox href=bigger-image-path.jpg title=emThis is the image caption,/em by someone”
 picture
  source media= (min-width:= 768px)= srcset=/image-path.jpg 1x, /image-path.jpg 2x data-category=Open-popup data-message=popupContent_bigger-image-path.jpg data-type=popupType_hiddenDiv data-fancybox-ready=true
  img class=image-no-border style=width:100%; height:auto src=/image-path.jpg alt=alt 

Please note the curly quotes after the 'by someone' in the title= attribute in the opening tag.

After testing this only happens when the title= in that opening contains html tags, like the tags it is currently containing.

This example isn't a one off, we have many examples of using html in the title= tags of fancybox tags, so I would like to understand why/how/where wordpress is making this change and a possible solution to stop it happening.