I had two independent cables from two independent rails connected. I also tried using the same cable. No difference.
Did you tried DDU, I test many graphics cards and when I install an AMD card after an NVIDIA I always have problems.
Must of the times I uninstall everything with DDU and the AMD works fine but one time with an RX 5700 XT, have ad to format the PC.
And after the format and windows clean install the card worked flawessed.
Please read the first post throughly and stop wasting my time. DDU is part of the recommended procedure by AMD. As per the first post: "I have even isolated my system and tried running Microsoft Flight Simulator 2020 on a brand new M.2 SSD with a brand new Windows 10 installation." Does this answer your question, sir?
My Asus TUF RX 6900 XT works perfectly
Asus Strix X570-E
Asus Ryujin 240
4x8Gb HyperX 3466 @3800
Western Digital SN750