Skeletal mesh Normal looks weir when it has animations with several different morph targets

Skeletal mesh Normal looks weir when it has animations with several different morph targets.

Here is the reproduced scene.

Video:20180718152541 - YouTube

I create three skeletal mesh with several different morph targets.

The 1st morph target makes sphere flat and Normal become flat too.

The other 3 morph targets(2rd~4rd) are just some single vertex movements or not moved. here are the 3 situations:
A.left—the 2rd~4rd targets use the same sphere(no deform).
B.middle—the 2rd~4rd targets make the skeletal mesh deforms not so obvious.
C.right—the 2rd~4rd targets make the skeletal mesh deforms lot obvious.

You can see B and C, vertex Normal is not so correct. vertex Normal should not change when the vertex is not moved by morph targets.
I think it’s some accumulation issue with lots of morph targets Normal calculations. Or UE just miss calculating the normal with morph targets vertex weights.