Not saying that at all - you may want to re-read my previous post.
The suggestion to restart (or unplug/replug) is to reset the HDMI chain (EDID issues abound) since you were stating you were getting everything as DD+ in Auto passthrough mode (you shouldnt be).
I also want make clear the Atmos you get from streaming services is DD+/Atmos (so it should be detected as DD+ by your AVR/soundbar, with Atmos). You should be getting PCM 2.0 as PCM 2.0, DD as DD, etc.
Put it in Auto passthrough and do some tests: NetFlix/Prime/Hulu/Disney+ output 5.1/Atmos in DD+, so all those should come through as DD+.
The Roku Channel and Apple TV output all (non-Atmos) 5.1 as DD. Paramount+ defaults to PCM 2.0, but can be configured for DD+ or DD output using Options (*) during playback.
If your AVR/Soundbar shows DD+ using Auto passthrough for all content (2.0/5.1) on all the apps/services I mentioned, then the most likely reason is that your AVR/soundbar (or whatever the Ultra is connected to) is applying some surround processing/upmixing, or alternately there is an EDID issue such that your AVR/soundbar/device is only advertising DD+ capabilities.
Highlight Settings/Audio/HDMI/Auto passthrough to see what codecs/formats the Roku detects your AVR/Soundbar/TV as supporting.