What's these Black triangular in foliage?How to get rid of them?

I made this tree in speedtree, and import it to UE4.You could see those hard edge black shadows.

My leaf geometry is like this

The center point is moved down so it is not a plane. And it seems that the edge of those shadows is following the edges in the center.

the resolution of lightmap is 512, so I think it is not a lightmap issue.

What cause this problem? How to fix it?