Image edits won’t update

Has anyone encountered a problem with image edits not updating in WP 4.9.4…?

I have inserted an image on a page and, in the admin, clicked ‘edit’ – specifically I want the image to have a ‘Custom URL’. so I get the image edit page and change the link dropdown to Custom URL and add a custom link (in this case a mailto:emailaddress)

I save the changes, and then update.

But out on the live page absolutely nothing changes – despite browser refreshes, emptying browser cache and even rebooting the computer, the alts do not take place. Other edits on the page, for example changing the text or such, work just fine. But alts to the image in the page are totally unresponsive. Tried about 5 times.

Are the image edits stuck in a hidden WordPress cache or something? Or should they be edited in the media section itself? (which rather obviates the image edit box on the page)

(* I’ve removed the offending image from the page because it’s a live site, so no point in giving a link to it)

Any help welcome.