PRIME Output Sink can't handle more than one 4K monitor by default

I am experimenting with PRIME Render Offload with the Output Sink capability in order to make use of the outputs on my laptop that are connected to my GPU. I have a laptop display (1080p) and two 4K @ 60FPS monitors that I want to make use of.

So far I have got the laptop and one monitor working successfully, albeit with some minor stuttering (sychronisation) issues. The problem lies in when I attempt to activate the second monitor at the same resolution. When I do this, xrandr reports “configure crtc 4 failed”, and in the X server logs I see that it failed to find a valid mode.

The odd thing is, if I reduce the resolution of the second monitor below 4K or lower the frame rate to 30, it will work. This lead me to believe there is something broken with the validation the driver is doing in this mode.

If I configure the X server device with these options and reboot, the problem disappears:

Option "ModeValidation" "AllowNon60hzmodesDFPModes, NoEDIDDFPMaxSizeCheck, NoVertRefreshCheck, NoHorizSyncCheck, NoDFPNativeResolutionCheck, NoMaxSizeCheck, NoMaxPClkCheck, AllowNonEdidModes, NoEdidMaxPClkCheck"

I’m using driver version 460.39 with a Quadro P5200 on NixOS.