Dominant directional light's material function disabled at high resolution

(Please note this is regarding UDK/UE3, not UE4 or UE5.)

I have a dominant directional light moveable. It has a material that simulates cloud shadows on the ground. It works fine at 1080. If I change the resolution to 4k, the light function disappears. When I switch back to 1920x1080, the light function reactivates.

I’m running the game in DX11 on an Nvidia RTX 3060, so I should have plenty of GPU power to spare.

Has anyone noticed an issue with light functions disappearing at higher resolution? How did you fix it?

I use a light function for underwater light caustics. Just tested and it’s working fine for me in 4k.

Not sure what may be causing yours to cut out. Have you tested in DX9?