When I use specified cubemap on skylight, the samplelevel result in reflection is not identical with the cubemap mip. I modified the mipLevel to 2 of GetSkyLightReflection() in ReflectionEnvironmentShared.ush to make sure it sample the miplevel 2, and exclude other lighting components to isolate the reflection.
Is this a bug or something? Cause it effects the roughness of PBR a lot.
UE4 Reflection on miplevel 2
CubemapGen Reflection on miplevel 2
UE4 Texture Display on miplevel 2