I am so glad I found this thread here because now I know the problem is with TV OS 15. Your assessment appears to be accurate since my system also doesn't support Dolby Atmos but supports up to 7.1 LPCM (through eARC HDMI 2.0b) in addition to regular compressed Dolby Digital 5.1.
I have a Bose Soundbar 700 with surround speakers and bass module that outputs up to 7.1 LPCM. Prior to updating my Apple TV 4K (2nd gen) to TVOS 15, I had no issues. All the Dolby Atmos/Spatial Audio tracks were playing perfectly well.
It took me forever to figure out where the problem was originating from since my Bose Soundbar, Apple TV and the eARC HDMI Splitter (HD Furry Arcana) all received firmware/software updates with just few day apart from one another. So I had no idea which one of the three updates caused the problem. It was just a really bad coincidence that has been annoying me for a while.
Just like everyone else here, when I play an Atmos track on Apple Music, my surround speakers and center channel (I think) are completely mute. It was completely by accident I discovered that the remedy is to either scroll back to the beginning of the song or just fast forward few seconds into the song. This problem doesn't exist with Lossless tracks. It only happens with Atmos tracks.
I initially thought it was my eARC HDMI splitter since it was the most buggy device in my setup. Just this past weekend, I finally upgraded my TV to LG G1 OLED TV, which comes with the latest HDMI 2.1 eARC (120Hz) ports. To my surprise, the problem was still there. That meant the issue was either from my Bose Soundbar 700 or Apple TV 4K. Thanks to this thread, now I know the problem is with Apple TV which I suspected the least.
I really hope it gets fixes soon because I absolutely love the Atmos tracks and how airy and transparent everything sounds. It's a great showcase what my system can sound like at its optimum.
Thank you all guys...