That was one of the first things I tried, unfortunately ;(
I've been doing some reading online for generic 3090 issues and there seems to be some that have issues with various PSUs with the 3090s due to power requirements. I've tried a few tools to undervolt but doesn't seem to help.
Now, I am not sure why other games do not trigger this issue but it seems a viable cause so I've ordered a new HX850 to replace my thermaltake 850. There are reports of various PSUs having issues (tho I couldn't find anything specific to mine) ... so I am hoping this is the cause. If that doesn't fix this I'll just have to deal with dx9 and 32fps in full screen mode I guess until the support folks respond to the ticket.