Ken,
what's going on is stranger than that : it looks like something fishy is going on with the "Label" field (I'm not sure if it's "Editor" or "Label", my MC is in french : "Editeur").
I just tested your hypothesis on an album for which all songs but two were "unassigned". All the unassigned" songs showed no Artist, Album, or Year info in MC, even though these fields were filled-in in the the song files.
I checked the "Label" tag on some correctly imported songs and on some "unassigned" songs also. All songs had "Grand Royal" as the label (no period). I deleted the Label field for all songs, updated the library and all songs were now classified correctlyl! Not only were the songs correctly comingled as an album, but all the previously blank fields were suddenly filled.
The only thing I did is deleted the "Label" Field.
I then tried to fill the "Label" field again with "Grand Royal" and this did not destroy anything. The whole album still shows as an album and all fields (Artist, Album, Year, etc.) are still showing.
Doesn't this look like a bug?