Cover will be deleted automatically

[MrPresident]

Add cover by file and afterwards click in any tag field (album, year - equal if filled or blank).
If you do not have saved the cover first, it will be deleted if you click in any other tag field.... try it
Adding a cover by freedb, MusicBrainz or from wherever.... does not fail.

[ohrenkino]

Actually, no modified data in the tag panel is saved unless you say so explicitly.
See e.g. here:

Leaving tag panel does not automatically save changes

When I enter tag information, all goes well - no problem. However, if I click on the filename in the right hand window, all the tag info I just entered disappears and I have to start all over again. I can duplicate this over and over again. The only way to make sure it doesn't disappear is to make sure you save it while you're in the tag so that you don't lose it. Depressing. Hopefully in the works for a fix. At least an autosave when clicking off the tag or something.