Left channel missing with FLAC stream from Victrola Onyx

When I am using the highest quality setting of my Victrola Onyx (FLAC streaming) to Sonos, the audio signal only goes to the right speaker of any pair. When I use the lower quality settings (where audio quality is unbearable, it's like going back to RealPlayer, for those who remember the modem era), both channels work. This happens with Arc + Play 1 surrounds, just a pair of Play:1s, etc. when I change the channel assignment (left/right) of a stereo pair, the problem remains: The speaker designated "left" plays no sound, the speaker designated "right" plays the audio stream.

Everything on latest firmware. My test above eliminates network as the root cause btw, please do not suggest replacing router, resetting switches, restarting devices etc.; that has been done, and network has been eliminated - because when you just change the designation of the speaker's channel from left to right, the problem follows.

Has anyone else seen this? Is this a known issue? Is there a fix?