It sounds like you've possibly got a PCIe 3.0 cable; but your motherboard, CPU, and graphics card are all 4.0. That will cause problems unless you configure the graphics card slot to 3.0 in the BIOS. 4.0 is double the bandwidth of 3.0, and needs a higher spec cable. There's no problem running 3.0 devices in a 4.0 system, or running 4.0 devices at 3.0 speed, you just need to avoid running 4.0 over 3.0 cables.
I'm successfully using a 4.0 riser cable from LINKUP in a build not that different to yours. Beware the variation in length measurements for riser cables; LINKUP exclude the boards and just measure the cable, so their 20cm cable is the same as the 24cm ROG Strix cable, for example. I picked them based on EK including their riser cables in one of their products, trusting EK to have picked a decent product, and I've not been disappointed with it, it's worked great and seems nicely made. CableMod have also started doing 4.0 riser cables, and I'd trust them too (but they were out of stock when I needed one).
I thought PCIe 4.0 and PCIe 3.0 had backward compatibility, but you would only run at PCIe 3.0 speeds...so I guess I got confused. I'll order a 4.0 cable and see if this issue persists. Thank you
Yes, PCIe is very forwards and backwards compatible, with the exception of riser/extension cables. Cards directly into slots automatically use the highest common speed between motherboard and card; so a 3.0 card in a 4.0 motherboard should run fine at 3.0 speed, and a 4.0 card in a 3.0 motherboard should run fine at 3.0 speed, with the lower speed normally being automatic. The exception is where you have a lower spec cable, as the smart stuff on each end of the cable has no way to properly detect that. If you've got a 3.0 cable and a 4.0 system, you need to configure the slot used with the cable as max 3.0 in the BIOS, then it will probably work. The same goes all the way back to PCIe 1.0, as far as I know, so 1.0 and 5.0 systems (and everything inbetween) should normally interoperate, but not when it comes to cables.
If you change your slot config to 3.0 in your BIOS, it will probably work, if that's the problem, but better all round to just get a 4.0 cable if you can.