Morph target normals

I have a problem animating a character’s full arm movement range from rest pose to “arms full up”. The morph target related to the “arms up” position should have different normals than the base pose. However FBX does not support per-morph target normals.
What is the ordinary solution to have per-morph target normals, or to import normals from a specific animation frame?
I tried using “Use T0 as ref pose” but although it can remove the problem with the normals, provided that the first frame is ‘arms up’, it produces alterations in the morph targets which have an unpleasant though subtle effect.
Why does this option distort morph targets anyway?