Thanks for the reply, Lovin'.
Let me explain what happened exactly, in more detail, because it kind of got buried in a couple of previous pages...
After the back-and-forthing with Treb in this thread and speaking with the Denon/Marantz Tier 2 engineer I got in contact with, it was established that by leaving the upmixers (both Dolby Surround and DTS: Neural) ON all the time, the 5.1 tracks would not be affected in any way playing back on my 5.1 system -- but they WOULD automatically engage when I needed them, such as for 2.0 channel signals (stereo or mono).
So, to reiterate, whenever I watch anything with multichannel content, the Denon shows this on the front:
Dolby Digital: "DD + DSurr"
Dolby TrueHD: "DTHD + DSurr"
Dolby Atmos: "DTHD + DSurr"
Dolby 2.0: "Dolby Surround"
DTS: "DTS + Neural:X"
DTS HD Master: "DTSHD + Neural:X"
DTS 2.0: "DTS Neural:X"
Now, I learned to accept this even though I HATE having to see the "+Upmixer" designation, as it is worth it to me for the upmixers to automatically kick in for stereo or mono stuff, but the original quirk I asked about was this: for whatever reason, when I stopped playback of a Blu-ray or UHD Blu-ray (but NOT a DVD), the Denon's front panel dropped the "+Upmixer" designation and just showed the "Dolby D," "DTS-HD," "Dolby TrueHD" logo/indicator WITHOUT the "+DSurr" or "+Neural."
See what I'm saying? I just thought that was weird, even though it didn't affect performance in any way.
Just recently, the AVR started leaving the "+Upmixer" on the display when I stop all discs, and I simply don't know why -- again, this is not a big deal, but I was just wondering if there's a bug in my unit (I believe I'm up to date with firmware). In other words, when I used to press stop on my Panasonic UHD BD player's remote after watching a Blu-ray or 4K disc, the Denon would drop the "+DSurr" or "+Neural:X" indicator on the front and just show "Dolby D," "DTS-HD" etc. NOW, suddenly, the display is LEAVING the "+DSurr" or "+Neural" indicator when discs are finally stopped.
I tried playing both Dolby and DTS sources and making sure both were switched to the AUTO decode mode as they played, to kind of "reset" the system, but it's still not behaving as it did in the past.