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Speedtree shading - wind causing black shadow clipping with wind turned on

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?
My light source is set to moveable.

Note that this flickering/clipping only occurs when the tree is moving. The underside looks fine when static. (Shown in third image).