Spatial Audio - Apple TV to home theater - stopped working in TvOS 15.0

I upgraded to a new 4k Apple TV specifically to enjoy Lossless and Dolby Atmos audio. It worked great at the introduction of the formats and continued to work great for some time. Then Spatial Audio just stopped working, giving me 1 or 2 channels of weak signals, although my receiver shows that it is receiving 7.1 channels. This seemed to happen around the time TvOS 15.0 updated.

Dolby Atmos streaming video content still comes in as 7.1 on my receiver and sounds fine, working just as it always did.

So, hardware is capable, but something happened to screw up Spatial Audio via the Music app.

I've tried all the usual: restarts, toggle settings, etc.