Why have my Displayports stopped working?

Hey I hope someone can help with this strange issue I am having where I can no longer use DisplayPort.

I have two computers, one had a Radeon R9, the other an Radeon R7, and both connected to separate 4k monitors with DisplayPort. They were both working great.

I replaced the Radeon R7 in one rig with the R9 from the other, and in the R9 rig I installed a brand new from box GeForce 1080.

Now neither system will boot at all with Displayport connected (using the same cables and monitors that was previously working). The GeForce system will still let you use Displayport after boot time but the new R9 system acts as if the Displayport is now broken on that card.

What is going on?

I ran DDU in safe mode on both systems for AMD and Nvidia drivers, I used MSITool to make sure there are no IRQ conflicts on the Nvidia system (the AMD system was fine).

OS Windows 10. Motherboards are both ASUS; a P5Q3 Deluxe with the 1080 and a M5A99X EVO R2.0 with the R9.

I have tried unplugging the monitors for a minute, resetting the BIOSes, disabling Fast Boot. Seems like there’s no way to get the DP functionality back.