You are correct. After removing that tag on all files, I see Roon handling everything just fine.
Edit: This is not actually the case. Roon still detects “.” as the Album Artist even after verifying it was actually removed, through a tag file editor, as well as verifying that the file modified date lines up with when it was changed. Roon persistently shows the previous (not blank) Album Artist tag for 154 albums.
