Hi,
I have gotten this problem occasional in my project mostly when using Static lights,
a solution that feels a bit wonky and temporary for me is to add some Stationary lights around the shadow shapes.
I mostly import from 3ds Max with Datasmith so these shadow shapes appears on my StaticMeshes
these shapes also can go over multibly meshes at once (making me think it got nothing to to with lightmaps)
things i have tried without effect:
- Overriden lightmap res (to higher
settings) - rebuild lightmaps
- tried same scene on different
computers (shapes still show up on
same spots) - Adding more light around shapes
- Problem happens even in different
unreal versions
The only thing that works for me is adding a Stationary light over the shape
Would be nice to know the reason why it happens, any help greatly appreciated 
