To start, please make sure that you're not editing the APEv2 tag of the file (disable reading and writing of APEv2 at Options > Tags > Mpeg).
Mp3tag support the Album Artist tag and displays it as BAND in the Extended Tag Dialog. That means that if you don't see it, it's most likely not stored in the ID3v2 tag.
You can also try to delete the tag and rewrite it via Mp3tag's undo feature [Ctrl+Z].