SkyLight Specified Cubemap Mip not identical

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.

227314-shot-2018-01-17-12-31-48.png

UE4 Reflection on miplevel 2

227315-shot-2018-01-17-12-31-57.png

CubemapGen Reflection on miplevel 2

227316-shot-2018-01-17-12-32-02.png

UE4 Texture Display on miplevel 2

Hello,

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Thanks