TinyMCE: can no longer add a link to an image in Plone 5.2.4

Add an image. Try to add a link to that image by selecting it and clicking the link button. Select item, close link modal. Link is not present.

I debugged this issue for quite a while the other way round. I have a migrated site with where the existing content has many images surrounded by links. I could no longer edit the image (change alignment, scale) in the image modal because the related item widget is half empty (there is a path but no image). Remove the link from the html, image modal works again.

Plone 5.2.4, tested this in coredev and multiple client sites. :open_mouth:

Further browser testing this morning: it only seems to happen in Firefox, not in Chrome or Edge, which may explain why I haven't noticed this before. There are no errors on the console in the browser.

Meanwhile I have found one Plone 5.2.4 project where the linking is working as expected, in Firefox, also Plone 5.2.4, but with a custom content type having the text behavior.