I think, WD SN850 might have some issues.
Western Digital will release a firmware update that eliminates a restriction in our product for this setting of MPS, expected to be available by July 12, 2021.
For optimum SSD performance, Western Digital recommends that customers download the latest BIOS version and drivers for their system and connect the WD_BLACK SN850 directly to the processor/CPU M.2 slot.
WD_BLACK SN850 customers can access/download the latest firmware via Western Digital Dashboard software. Once the firmware update is available, customers who open the Dashboard software will be prompted to make the update.
Just check you have latest firmware installed for the M2 via WD software. Yes, Top is CPU M2 on this board.
If this all check out and does not solve the issue,
RAM - let's talk about it. Are you running DOCP DDR4@3200 with it ?
Only ever on the ram, the version number, and like I mentioned it can vary a lot just by version number, has in the past. Anyway, you mention tested ok.
Do you have both E-ATX connectors (top left of motherboard) installed (it's long shot but deos not hurt to have both connected and yes I know what manual says and why there are two).
Only other things to consider is overtightening of the cpu heatsink, or motherboard standoff screws or screw in wrong place, and backward cpu backing plate (if changed). Which I ruled these all out based on your first post, so not really sure about the cause here.
I'm out of ideas thereafter.