Right, FBX does not support custom vertex normals on morph-target model, but the 3d editor does not support them. I just wrote about this in a previous comment. Perhaps it would be a nice feature but not required for this task.
From my experiments with geometry attached in the first comment should be: Inside the engine the normal model with the morph, in a bind pose visually coincides with the normal to the editor, there is a visible difference when you play the animation. Also, I tried to import the model back into 3DS Max, any problems with the normals I had not seen, normals are exactly the same as the intended.
therefore - the normal morph geometry match until the model is in a bind pose, after which the skin shader should be all incoming data for bone transformation matrices (vertex coordinates, bone influence and the normal has with Morpher), but something goes wrong .
Theoretical results must match the result in the editor (as the initial data are the same) but this is not happening.
Throughout this chain for me it remains unclear the calculation of the normal for morph. The fact that changes morph vertex normal and the editor and unreal. Theoretically, the normal should always be tangent space. But the small difference between the editor and the unreal is still there.
I’m not sure, but I think it’s a mistake in the calculation of the normal vertex inside skin vertex-shader.
(I try to check that translates “on Google Translate”, so I apologize if there are any inaccuracies.)