How do you know that a non 7.1 track is playing when Plex shows a 7.1 track playing? What does your dashboard (and receiver) say is being played?
What order are the audio tracks in? For example the Xbox has a BUG where if a DTS or Atmos track is not the first track there will not be any sound from that track. But other tracks would play just fine. It took me a long time to figure this out and even longer to rearrange everything to make it work.
