You are correct. However, Atmos shares the same codec as Dolby Digital Plus. What's missing is the Atmos extension on the 1080p stream.
BTW: This issue also occurs on Peacock's Atmos 4k streams as well.
Hey @IanG,
Thank you for posting here in the Roku Community!
We want to investigate this issue further. Is the problem specific app, or is it occurring on all apps?
We'll wait for your response.
Thanks,
Roku Community Team