I think I may have noticed a bug when it comes to copying a cover from an existing MP3 with tags to a new untagged MP3 when using the Copy Cover command.
Sometimes I will need to add or replace an MP3 in one of my own albums, but I find that when copying a cover from another file already in the album, and then pasting it to the new file and saving, it appears to work but then when all files in the album are selected, the cover preview goes blank and says "cover varies".
I found a way of solving the problem by extracting the cover from a file in the album to folder.jpg, and then adding that to the new file - then it works, the cover doesn't vary in the album at all.
At all times, it is just the one cover image in all the files.
I don't know if it's a bug, but since last night I've discovered another way to copy all tags over to replacement MP3s, and that works as well, so it maybe doesn't matter. But I just thought you ought to know.