using 4.22.
I have two meshes for a skeletal, left arm and right arm with their own morph targets, created in blender.
Brought into unreal and combined the meshes. The skeletal showed the morph targets at zero value.
however the left side seems to be distorted when right morph is at 1 value. Similar for the left morph.
So how can i prevent this from happening?
Likely, the morphs in blender have those vertices assigned.
is there a reason you cannot properly combine the mesh in blender with the correct morphs?
That was what i thought as well as morph should take ordering of vertices to change/retain transform. But if changing the left morph should technically move the right morph as well instead of slight deformation.
Anyways, I have solved the issue. It was lack of understanding on how blender works. 
The deformation was due to the last pose that the armature was in before exported as fbx. So the “basis” key in blender for the shape key didnt do anything to capture the original pose.
Thanks for helping.
