Attempting to blend SkyLight Cubemaps via the SetCubemapBlend function to blend between different Cubemaps to make the changes seamless is not reflected.
As a workaround, in LumenTracingCommon.ush, replace GetSkyLightReflection with GetSkyLightReflectionSupportingBlend to apply the blend. This is also mentioned in the following thread.
When changing the value of InBlendFraction, only A or B is reflected and not blended.
When blending via the SetCubemapBlend function, A and B are blended according to the input InBlendFraction value.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-198199 in the post.