This can happen if, once you've entered the artists for the individual tracks, you go back and amend the 'headline' artist's name in some way. It cascades that change right down the entry.
I've had a look through, but I can't discern anything obvious. As you suggest, it's not mono/stereo, and I don't think it's genre either. Price code would look the most obvious explanation, but I have nothing but deduction to work from on that.