Hey, I’ll be just wild guessing here, but this could be because UE is trying to compute shadows based on the geometry, rather than texture (your leaves are textures on quads as is common I assume).
Try looking here: How to make tree shadows with leaves alpha also shadowing? - Rendering - Unreal Engine Forums
or
https://wiki.unrealengine.com/Two-Sided_Foliage_Material
Or try looking into translucent shadows (that’s used more for colored glass and some such but it could give you some pointers still)
Also UE 4.10 is a relatively old version, if this doesn’t help, you could always try to upgrade to a newer version that tends to offer improvements with rendering. Hope it helps some.