I also have this issue, this is a core issue inside Unreal 5.6, I tried everything, migrate, clone, new project, change cvars, project settings, change shadow methods, raytracing (cached, hit lighting, hit lighting for reflections), nothing worked, this a problem inside Lumen:
UE5.5 Left - UE5.6 Right
Look how pleasant the colors are in UE5.5, and how the colors break in UE5.6 (Lumen scene)
