Why is Apple Music playing some music in stereo when it's marked Atmos?

I just bought an Apple TV box hooked up to a Dolby Atmos soundbar. Atmos decoding works, comes through 100% with the TV+ app.

But with the Apple Music app, about half of the music marked as Atmos is sent through as 2 channel stereo instead, while the other half is output properly as Atmos. It seems totally random. Obviously it's NOT my TV or soundbar because Atmos is getting through fine mostly, especially in the Apple TV app, so please don't tell me to go & check my TV/soundbar settings, it's just in the music app that some but certainly not all music marked as Atmos on the screen comes through in stereo.

Also, I'm aware some albums only have a few tracks in Atmos and the rest in stereo. That's not the issue. This is music clearly marked with the Atmos logo on the screen and which does play back in Atmos on my Sonos Era 300 speakers. Just not via the TV box.

It's disappointing because the ability to play back music in Atmos via the streamer is 80% of the reason why I bought it in the first place.

Is this a known issue? Anything I can do about it?