WD Support came back with the following…
The fact that the drive works with the intended speeds in one of the slots but not the other points out that there is some sort of issue with that specific slot of the motherboard.
You can put the drive in that slot and use WD Dashboard to make sure all settings are up to par.
Furthermore, I can request the compatibility list from our engineering department if you wish, as it is not readily available
So once again, they ignore the confirmation of the problem across several motherboard brands. I have requested a specific list of supported GEN4 motherboards when using a PCH M2 slot and as you can see, apparently that list is not readily available…for some strange reason.
The more this goes around, the more its becoming clear that WD are attempting to cover up a hardware level incompatibly on the SN850.