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.