Speedtree Shadows black with Wind

Having a strange issue with either - Normals, Subsurface, or shading of some kind.

My tree looks fine in Speedtree 8.4 - lightmaps are perfectly green and good to go.
Importing with created materials checked etc.
Trees look more or less fine at a standstill in UE4. As soon as I add a wind volume, the movement in the fronds causes the frond meshes to clip in and out of black. Seems there’s some issue with the underside of my frond meshes?