Well, I designed it this way.
When you detach a face to another part, you do not always want this face to get the face behaviour of the target part.
So face is detached and keep his own behaviour.
And if this behaviour is different than the behaviour of the other faces in the target part, then global target part behaviour is displayed as "custom" to show that part is made of faces with different behaviours.
The custom behaviour is not a GP4 behaviour at all, it's just a flag I introduced in GP4Builder to say the part had different faces behaviours and that's the face behaviours themselves that are interpreted by the game. So no need to remove custom each time but if you want to convert your grass faces to tarmac but then, that's your choice.