Model material remapping not working when FBX model hierarchy is changed

I import a vehicle model (FBX) It has ~50 child objects and 5 material slots.

When I add the model to a scene (without changing the hierarchy) and add the materials to the models remapped materials slots and apply all child object get their materials properly.

Hierarchy example:
Car
-Left door
-Right door
-FL wheel
-FR wheel
-RL wheel
-RR wheel

If I add the model to a scene, change its hierarchy, create sub groups (like doors and wheels) then move some child objects to those groups unity gives a warning, saying “This action will break the prefab instance…” although it’s not a prefab. I continue and arrange the model hierarchy. And then, when I add the materials to the models remapped materials slots and apply, all child objects get material missing warning in their renderers and turn to purple.

Hierarchy example:
Car
-Doors
–Left door
–Right door
-Wheels
–FL wheel
–FR wheel
–RL wheel
–RR wheel

I need to be able to change the model materials after I arrange the model hierarchy. I can fix every child object one by one from their mesh renderer component but it’s not the correct way as there is only 5 materials but more than 50 child objects. It takes a lot of time and prone to mistakes to change them all one by one.

I’m using Unity 2018.1

Is this a Unity bug or am I doing something wrong?

Is it possible that the “Doors” Object has the material? In some 3D Softwares a child automaticly take the material from the parent if no other is assigned. And when you move them in the hyrachie it looses the “material link”…