NEVER had this on any other build . I start a 5.1 Flac to my Oppo103 and it sometimes is stereo , amp says PCM 5.1 . I restart the track or scrub it forward or backwards a little and it pops into 5.1 .
Haven't changed a thing EVER on JRiver , and is always set to source number of channels
If I ignore the stereo down mix the next track usually pops back to 5.1 all on it's little lonesome
I'm streaming via Mini Mac 2018 over ethernet wire to Oppo103
The Audio settings for the player zone in MC are NOT used when playing to a network device.
Since you are sending it using ethernet (via DLNA) there are other things to check.
Look at your DLNA server settings in MC and make sure the format is set to "Original Format".
If that is OK it's possible your Oppo has gotten confused, power cycle it (remove the cord, don't just turn it of and on via the front button).