some shadow problems about grass Foliage

Hi guys, here i have some questions.
I paint some grass meshes with Foliage,and some shadow probles appear after lightbuilding.
If i don’t build any lighting ,the shadow seems good…so strainge,No building is better?
Last I give up Foliage and begin use staticmesh and piace them in level manually,it is all right after baking
who can tell me what it is like this.Thx a lot!

Perhaps it’s realm time shadowing before you calculate shadows, that could explain why it would be ok ?

I had some probs with foliage too, Cascading Shadows on the directional light helped