Pretty much what’s in the title. For no discernible reason, having higher accuracy CollisionFidelity on Unions and MeshParts in any lighting mode will cause strange light sources and shadows to appear on and around the Union/MeshPart, where brightness increases with collision accuracy. This occurs on all lighting modes and is most noticeable at night when both PBR sliders are set to 1. When PBR sliders are set to 0, there are now very dark shadows where the bright spots were.
ShadowBugRepro.rbxl (43.4 KB)