I’ve encountered some strange things with shape keys and skeletal meshes I can’t explain.
I’m using a Dual Quaternion Build of 4.21, but it’s the same, although to a lesser degree in the official 4.22 version as well.
Exported shape keys from blender (2.79) cause strange shadows in UE4, no matter the mesh resolution. At first I thought that my former mesh had a too low quality, but that was not the culprit.
The shadow doesn’t correspond to the mesh shape. It looks like there were holes in the arms, but there aren’t any holes. It seems to be some kind of lighting problem. It’s at the same location, where the Morph Target has been applied. (removal of armpit bulge)
The arms of a posed character in blender look smoother than in UE4. In UE4 there is a strange distortion (red marking). I can’t explain it, because I don’t even use any modifiers in blender, which could have caused problems with UE4, only the Armature Modifier. It looks as if the bone weights in UE4 and blender differ.