The Plex Android TV app does not support multi-channel audio only files.
See this post (FireTV devices also use the Plex Android TV app): Amazon Fire Cube not playing multichannel audio properly - #11 by DaveBinM
The last time I tested multi-channel FLAC (July 2020), Plex played the left and right channels and dropped the rest: Why no support for 5.1 surround music playback? - #9 by FordGuy61.
