Hi and welcome!
I think the issue you're observing is due to a bug in iTunes/Music.app which can be observed with the version of Mp3tag you're using (v1.0.2).
With this version I've changed the encoding of the cover description to match the global settings from Preferences → Tag Formats, which was UTF-16 in your case.
From the changelog:
Fixed: ID3v2 cover description only supported characters of ISO-8859-1
Unfortunately, it became clear quite quickly that iTunes has problems with this (— although it's expected this way according to the ID3v2 standard and I've considered this a previous bug in Mp3tag).
I've reverted the change with v1.0.3 of Mp3tag for Mac (released March 3) to increase compatibility, so it should be working again if you re-save the tags (and possibly re-add the files to iTunes, if the changes are not picked up).