Well, that's weird. For good measure, you might check the ID3v2.1 and the APE tags in the tag inspector, too. At exactly which point is this tag appearing? What are you doing when that happens?
Since it's in the Display Artist tag, it's less likely to be coming from outside MusicBee, but I can't think of any reason it would be from inside, either. Do you have the Additional Tagging & Reporting Tools plugin installed, by any chance?