Hey there!
I have been using UE4 4.27.2 and have noticed that every time I render using GPU Lightmass there are discoloured stains in areas where the light isn’t hitting directly. I am using a static skylight.
I use high GI samples and high lightmap resolutions and still get these patches. See the blue dashes for the colour change on the ceiling and the red circle for the rainbow patch.
Does anyone know how to fix this please?