Apple Music is not purging previously played songs after grouping then ungrouping speakers. Certain speakers will not work and a message, Playback Stopped In Use Somewhere Else, appears. This only occurs with Apple Music not Amazon or Sonos sources. Work around by ungrouping all speakers then manually regrouping beginning with the non-functioning room. This is where I see the music played prior to grouping has not been purged.
This problem is unique to Apple Music