In some FLAC files the field 'Album artist' is empty, but when I look onto the tags with another tag editor (MusicBrainz) I see there is data in a tag names 'album artist' (note the lower case first letter). Some files have both 'Album artist' and 'album artist' and it looks like mp3tag only handle the one with uppercase first letter. Are I am missing something here ?
What does the extended tags dialogue Alt-T show?
The tag field should be called ALBUMARTIST in MP3tag.
Extended Tag Dialog shows the 'missing' field. Guess I have to 'play' with the 'Adjust visibilty of the columns' dialog ?
I would use a mapping to map ALBUM ARTIST to ALBUMARTIST.
See Options Ctrl-O Tags>Mapping.
and see here: