Atmos with Nvidia Shield but NOT with Apple TV 4K

I had previously been using the Nvidia Shield, but just picked up a new Apple TV 4K. I had been enjoying Dolby Atmos with my Sonos Arc setup + the Shield; however, with the Apple TV 4K, I do not have the option to output to Atmos anywhere, not from within settings nor from any streaming app.

Hardware setup is identical (in fact I hot-swapped the HDMI connection and am using the same port). Titles in Netflix or Disney+ that normally say Atmos on the Nvidia Shield say 5.1 on the Apple TV and when I check system info on the Sonos App while playing something, it is indeed outputting to 5.1.

Does anyone have any ideas? When I switch back to the Nvidia Shield, I once again have Dolby Atmos (no rebooting required - although I rebooted all hardware just in case that had anything to do with it).

My current setup is as follows:

TV: TCL Roku 55" 55R615. HDMI modes tried: 2.0 and Auto. Audio Formats tried: Passthrough and Auto.

Audio: Sonos Arc connected to Arc HDMI port 3 with SUB + 2 rear Ones.

Streaming Box: Nvidia Shield Pro 4K / Apple TV 4K latest gen - Swap between the two on HDMI port 1, using same HDMI 2.2 certified cable.