Tagging and cover art is quite a difficult thing in ID3v2.3, since the standard allows some manipulations of the bitmap or the whole tag that are not easily recognizable when dealing with text tags only.
MP3s have small parts of music information all starting with FF FE (just a rough example, it’s a bit more complicated). To avoid that MP3 players that do not understand the ID3v2 tag would accidently think an embedded image would contain music, all these FF FE occurences inside the image (or the whole tag, depending on ID3v3 vs. 4) CAN get replaced, if a special flag is set. Some taggers ignore this flag, which usually destroys a lot of the tag, maybe even a few seconds of the music.
Btw, while TagsRevisited is kind of bad to use for the whole collection, I hope you had at least seen the “Recursive” option in the lower right? Would have allowed to do everything from artist to artist instead of going into each album at least.