One of the stages I was just on had only one of the parts visibly filled (upper), yet the only option available to me was to join on the middle part. There was another person there presumably trying to join the third “empty” spot, but they gave up and left after a few songs.
During the songs, the notes came towards the empty spot as if there were someone standing there, but since no one could load into that spot to play said notes, the bar on the right would stay completely empty. It remained this way for the rest of the instance – no one could join, but the game acted as if the spot was already filled.
