Using Mediamonkey and Mp3Tag-mapping you would have to create a mapping pair like this:
Tag: Vorbis Comment
Source: ALBUM ARTIST
Target: ALBUMARTIST
So every flac file in Mp3Tag shows ALBUMARTIST if their is a tagfield "ALBUM ARTIST". But it also shows 2 ALBUMARTIST fields if there are "ALBUM ARTIST" and "ALBUMARTIST" in the tag.
The mapping is very useful if your are working in Mp3Tag across different formats (i.e. MP3 and FLAC). Without defined mappings you would have to create different scripts and format strings for the formats.
Mediamonkey treats ALBUMARTIST in flacs a little bit confusing.
When it finds a field like ALBUMARTIST or "ALBUM ARTIST" it manages them as a pair. As soon as you do any writing with Mediamonkey and it finds either one of these tag-fields, it creates the other one automatically but it displays only "ALBUM ARTIST". If you delete the shown "ALBUM ARTIST" with Mediamonkey it deletes also ALBUMARTIST.