Regression since 4.13 due to change in UStaticMeshComponent::InvalidateLightingCacheDetailed
Open QAGame editor with the default new level
Increase the floor XY scale to 100x100 to make a large flat area
Open foliage tool
Add a mesh as, eg Barrel_02
Set the brush size to 2000
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-40221 in the post.