I've noticed a very strange bug. When using the foobar2000 player and alterting tags with it, MP3Tag will no longer read tags from the altered file(s). Now, I don't think it's an issue with Foobar itself as, after altering the tags with Foobar, I've checked the files in both Tag&Rename and The Godfather. Both of these tag editing programs read the tags fine.
I know fixing this will probably require huge changes to the ID3 (I didn't try it with APE tags) reading library thingo you use now, but I really prefer MP3Tag to all the alternatives.
Maybe foobar2000 is set to write APEv2 tags only and MP3Tag only reads the ID3v2 tag. Check out your read and write options in MP3Tag, as well as foobar2000 (Preferences -> Playback -> Input -> Standard inputs).