I have even used gpu-z and got the same under load or not.
Things I tried:
-went into bios and tweak pch configuration
-moved m2_1 to m2_2
If no have already bios 0816 , try put m2 in a slot 3-4-5 and bios update so you get alsoget board reset https://dlcdnets.asus.com/pub/ASUS/mb/BIOS/ROG-STRIX-Z790-E-GAMING-WIFI-ASUS-0816.zip?model=ROG%20ST...
I would not RMA until you troubleshoot a little more. Have you tried taking out the m.2 altogether or moving it to another slot like Ghigo recommended?
And does the video card do x8 in another slot? Can you try the video card in another PC?