I did some searching, read up on "Carnac" and I think there is some sort of bug.
I imported a bunch of albums, and some individual files with existing tags, ended up with their database fields for "artist" and "album" changed to something different than the tags.
I checked the raw "Vorbis Comment" data and in each case, there is complete existing tag information.
In each case, the track number and genre are successfully imported.
In each case, the folder name is used as the album, and the parent folder name is used as the artist. In actuality, if I actually used that scheme, I might not have found the problem (and the fact that it is not previously reported might be due to it being a popular scheme).
However, my folder names are "Artist - Album" and the parent folder is "_Genre", so I immediately noticed that some tracks had "_Jazz" and "_Classical" as their artists.
Hmm... I think that the track is the last track in the album in each case...
UPDATE: see later in thread...