So after hours of research I figured out.
That it’s a known bug that won’t be fixed:
for more details i found this thread:
https://answers.unrealengine.com/questions/280561/morph-messing-with-smooth-groups.html
or here:
https://answers.unrealengine.com/questions/456613/モーフィンク時にモーフターケットの法線か適応されない.html#answer-463657
So if you want to have this kind of sharp edge you will need to change your geometry.
I hope it can help.