Well there were/are several people saying about it in bang4bucks comments and on that big reddit thread then factor in the same showing on other peoples youtube footage too. I do agree though, there are a lot of factors but generally when you get that many people all showing the exact same issue, it does look like something isn't quite right.You say several other people, but really only one YouTube channel showed some examples (outside of DF and the UE5 alpha demo).
There's also no context provided, no information on settings used on each system and steps to reproduce. Also he was capturing videos using external hardware, there's so many variables there. Everything is vague with no substance, which is always a red flag.
You can be sure that Digital Foundry would be all over it if this was a genuine issue in games I can promise you that.
I also just remembered you benched the 3090, 6800 and 6900xt, had a quick look and looks like you were suffering the same issue too:
However, your latest run with the 6900xt:
So seems it was a game(s) or/and amd thing.