Using Mp3tag v2.86, if I select a bunch of MP3 files with different covert art, the left tag panel states "Cover varies". Then, if I right-click the cover art box and click "Remove cover", the text "Cover varies" goes away and the removal appears to be pending, but if I click away or use the arrow keys (with the Options preference "Save tags when using arrow keys/single mouse click" enabled), the removal is not actually applied. In contrast, if I do the same thing using the right-click "Extended tags" menu and click OK, it performs the removal.
Modifications that you entered via the tag panel have to be saved either with Ctrl-S or the save icon in the toolbar.
If you want to save modifications when you navigate to another (set of) file(s), you have to set the option in
Tolls>Options>Tags: Save tags when navigating ...
Yes, I mentioned the bug occurs with that option enabled.
I just tested it as follows:
Set the option to save on navigation.
Select at least 2 files
Clicked on the image field in the tag panel and selected "Remove cover"
The display changed to show no cover any more.
I clicked on a different file,
a progress indicator message opened to show that tags were saved in the amount of selected files.
Checked the files individually: non of them had an embedded picture.
I did the same thing with files that had different embedded pictures - and you are right: the pictures do not get removed.
Confirmed. Thanks for reporting!
This is now fixed with Mp3tag v2.86b.