Have you looked at the file infos in itunes? itunes uses a checkbox Part in compilation to mark albums with multiple artists. itunes does not use an album artist tag.
You can change that also in Mp3tag if you use ALT+T and add a tag ITUNESCOMPILATION with the value 1
But you need the 2.32t version to do that. (but only use this version for mp3 files, not aac)