Denon / Marantz remembers audio settings based on the combination
of Input + audio format (DTS, PCM, TrueHD, Dolby Digital+, 2ch, etc.)
So If I play a Multichannel Music BluRay, like the awesome Steven Wilson 'To The Bone' using the TrueHD 5.1 and click the 'Music' selector on the remote, and then select TrueHD + Dolby Surround, I will get an upmix to 5.1.4 using the DSU 'music' mode.
If I then throw a BluRay movie in the player and select a TrueHD 5.1 soundtrack, it will playback with the same upmixer settings as before, as the input and the format remained the same. But in this case, the upmixer mode is not quite right, so one needs to click the 'movie' button, and then select TrueHD + Dolby Surround to now get an upmix using the correct movie mode.
Next, if I put a regular CD in the player, and I had previously selected 2ch for that type, the preamp/receiver will go back to that setting for that format, so I get plain 2ch output from that CD. Nothing else to do.
And finally, if I put in a BluRay movie with an Atmos soundtrack, the unit will automatically goes into Atmos mode with no further user input, as there are no other settings to mess with for that format.
But it sounds like your issue is due to the way Dolby Digital+ streams are handled, as to the unit, that's the same input and audio format (Dobly Digital+), so maybe that's why it's requiring a manual selection step.
What streamer device are u using (TV app, FireTV, AppleTV, etc.) and what streaming service?
I have a Denon X3600 with AppleTV 4K and FireTV 4K, Panasonic UB820 in one system, and a Marantz 8802A and same set of players in another and other than the need to double-check that the DSU mode is movie vs music, I rarely have to mess with anything.
My challenge is often getting the streamer or service selections right so I actually can get Atmos