PC Master Race

Does your adapter happen to have a PLX chip? Those are designed for this very specific use case. The adapter is designed to run on 8 lanes and the PLX chip handles the rest, so GPU in one slot and adapter (with PLX chip) in another slot, x8/x8 biggity bam. Right now you're tryinf to run it x8 and those 8 lanes are just going straight to 2x M.2 as x4/x4, but PLX would divvy it all up essentially 4x M.2 at x2/x2/x2/x2, that kinda thing. If this adapter doesn't have it and you can't get this to work, and you're willing to blow another hundred bucks, that other kind of adapter would def work.

Without knowing your motherboard I'd def download the manual and ctrl+f some promising keywords such as 'bifurcation' or 'PCI' and basically hunt for anything in the manual that describes settings to enable this. Sometimes on a 3 slot board the bottom 'chipset' slot can be toggled to run something different than PCH, like some other controller.

I'd also try running the adapter in every possible configuration, on the most current supported PCI-E standards and a previous generation if motherboard allows, ie: adapter top slot, GPU middle slot, GPU top slot, adapter 3rd slot (if mobo has one), disable PCI-E 4.0 or enable 3.0 each time just to see if some configuration works (kinda tedious but costs 0 dollars so why not)