I am on a 2019 Pro running 10.15.7 - Yes, it does almost seem like the monitor is trying to find the right resolution to display (just a guess).I have not had this problem, and it doesn't sound particularly good to me. What machine are you connected to? It seems like I've seen these kind of resolution / placement oddities when connecting externals to laptops, but I don't think it aught to be happening in any overt way w the Pro or compatible machines.
The monitor firmware is still: 3.3.23 as I haven't received the option to update yet. I've read the above posts about disconnecting and powering down but I am reluctant to check the "Automatically keep my Mac up to date" box even though I don't have the "Install macOS updates" box checked in the advanced section (I'm really not ready to upgrade to 11 yet).
I turned off the "Put hard disks to sleep when possible" options a while ago with no change in behaviors. I have the internal 8TB Promise disk for Time Machine and I thought maybe the computer was waiting for it to spin up before waking up the monitor but I don't think that is the case.
Just this morning I clicked the mouse just once and counted to 6 before the screen came on, so it doesn't seem to be an issue with the bluetooth connection waking up the computer.
I may actually get the extended monitor AppleCare for the first time ever...