One of my endpoints runs on a Raspberry Pi 3 with A Hifiberry Digi + HAT. I only get stereo when playing FLAC 5.1 files. Hifiberry says that it outputs 5.1 but they only reference kodi. I am running the Roon Bridge on the Pi.
I tried changing the Multichannel mixing parameter in roon to “channel mapping only”, but it doesn’t give me 5.1
If you have this working, can you cast some light on the settings you use?