Updated to the new Nvidia 340.43 driver this morning, and it immediately broke all of my UE4 projects and builds. The problem exactly was that it caused the sphere reflection captures in my scenes to show as if it was 0 roughness for every single object and material, then flicker back to what it should be for a few frames before going fully shown again. It didn’t seem to affect screen space reflections, turning them off had no impact on the flickering, where if I removed the reflection captures the flickering stopped entirely, even with SSR enabled.
The problem resolved itself the moment I reverted back to the 337.88 driver, and the problem existed on the Github compiled build and the build downloaded from the launcher. Reinstalling and recompiling the engine seemed to have no effect.
Possibly relevant computer specs:
EVGA GTX 780, i7-4770k