After having so many audio issues with my previous Ultra I really thought that Roku finally got it together when they came out with this new "upgraded" box.
After following everyone's suggestions on different settings I was content that my "only" issue was no audio on Peacock commercials. The sound was great on everything else and I had no video issues.
Now I recently noticed that a new show, The Flight Attendant, was streaming in Dolby Atmos yet my Roku Ultra was only passing through Dolby Digital +. I was getting Atmos before through the box when appropriate.
I can't wait to see what issues will start happening next. They dropped the price on the new box after I purchased it, don't respond to support issues and apparently do not have a fix/update to help us.
Considering that "The Flight Attendant is an HBO Max Original, and Roku isnt carrying the HBO Max app (or content) yet due to the lack of a deal with WarnerMedia, I wonder how exactly it is you are watching the show on your Roku.
If in fact you are using Screen Mirroring or AirPlay2 to watch it on your TV via the Roku, that would probably explain the limitation.
Otherwise, I'm not sure how you were watching this HBO Max-specific show (what app?).
Also, for the record, Dolby Atmos is carried in 4 ways: DD+/Atmos (streaming services use this), DTHD/Atmos, MAT2.x/Atmos, and AC4/Atmos
(Most) Streaming services use DD+ to deliver multichannel audio, and that includes Atmos (DD+/Atmos), due to DD+ low bandwidth optimization.
So, it would be normal to see DD+ and/or Atmos indicated on/by your AVR (depends on the manufacturer/model) coming from streamed Atmos content from a streaming device.
Content (and apps) has to be specifically Atmos tagged (in the app) & encoded (in the content stream from the server) in order get Atmos in "Auto passthrough" mode.
If you want to convert/transcode all audio output to some form of Atmos regardless (not recommended) as supported by your AVR and detected by your 4800, then you would choose "Auto detect" mode.
Also, FTR, there was a Peacock TV update a day or two ago (1.7.76) and there are routine updates to the ad services/libraries - might want to check Peacock TV and see if the problem persists.