After messing around with a lot of xrdp settings over the past few days - with no success in solving the problem, I found a workaround!
I saw a section in the blog post Ubuntu 18.04 – Detected issues with xRDP on Ubuntu 18.04 that described how the ability to log a userid in to both a (physical) "console" session and an RDP (Remote Display Protocol) session had been removed! Here is specifically what it said:
It used to be possible to have the same user connected locally and remotely (even do the session where not the same). This capability has been removed from some time now and we do not why and if it’s possible to implement this feature back (we have not checked that yet).
So after seeing that, I created a new userid and logged it in from Windows-10 using RDP, and it brings up a new MATE desktop perfectly now!
I'm still not sure why I was still getting a blank screen when I tried to login my RDP session directly as "root" earlier, but no worries - I didn't really want to do that past simple debugging anyway... I still don't understand why I was able to logon to both my physical console and an RDP session on my 4GB Pi-4, but not on my 8GB Pi-4 in the first place - when both were running slightly different versions of Ubuntu MATE 20.04.x. I'll have to try to diagnose that when I get my 4GB Pi-4 reconnected up again.
I hope this works for others and that at some point it may be possible to put out a better, more specific error message if an RDP logon is done for a userid that is already logged onto a physical console. That would have saved a lot of time and trouble tracking down a workaround!