1. Version 20H2 (OS Build 19042.804) - Windows 10
3. 6.1 (trial thus far, though I'd imagine that would not affect the problem)
4. All default. Note that I also have Fences (latest version), also in its default location.
I mention it in case it could be the culprit
5. More complicated so I'll answer below
First, originally I never had the problem. It only started when FX came along.
The problem is random. That said, when FX is installed and running it happens about 85% of the time. I disabled it to no avail. I closed it down, right down to the task manager. No luck. As it was on trial I reported it to Stardock, to no avail, so I removed it. Did the problem go away? No. However, it happened about 45% to 50% of the time. As it never happened before, FX changed something somewhere -- which is why in mentioned Fences.
I also noticed that FX did not restore other things. For example on some minimized windows they don't minimize nicely but just disappear instantly after I removed it. Windows became a bit odd but maybe the culprit is around there.
Due to that oddity I decided to give FX another try to also get a bit of normality. I also thought it best the make sure the issue is addressed by you. I have been a customer for years and used many of your solutions, starting with Window Blinds then Object Dock (which I replaced with Nexus as it works better since issues with that were not resolved).
I hope this helps you find the problem!