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Seems to be an issue with the indirect lighting cache where if you have interpenetrating static meshes (for example static mesh rocks that make up the ground), there are cases where the cache points along the boundary where the static meshes interpenetrate are spawned inside one of the static meshes, and as a result the sample at that point is very dark
Volume Light Sample Placement Scale is 0.2 here
(The project in question seems to have this set low as well to get many samples)
Seems more likely to happen with a higher number of samples?
If the character walks into this area, their lighting suddenly becomes very dark
Was able to repro this in 4.8
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-16837 in the post.