Unreal forgets my material assignments

So I have a car from a datasmith import in my scene
I drag another material onto the car.
I save the scene, I even apply changes to my car instance.

I build the scene or I reopen it. Mateiral assignments GONE.

What happened to UX PRINCIPLE 1: “Never loose my stuff”?

Why does Unreal forget/ignore my material assisngments? (And why is it so cumbersome to work with imported 3D models anyway)