Potentially missing lightmass path for skylight that needs to imitate the static directional light GI solution.
Download + launch the licensee issue from here - [Link Removed]
The issue is immediately visible, but for comparison, change the scene's skylight to stationary/moveable and see it immediately brighten. When switching back to "static", the scene darkens again even if lighting is rebuilt.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-127661 in the post.