Child order is forced in my prefab

I have a prefab which looks like this

A(prefab1 root)
- B(an empty game object)
-- D(prefab 3)
-- C(prefab 2)

D,C is child of B. I want the sibling order to be C,D.
I can (of course) drag&drop the gameobject in hierarchy to do this and everything looks good.
But when I exit the prefab view, the sibling order becomes D,C again. No matter what I do, it seems prefab3's instance is always placed before prefab2's instance when exiting prefab view.

Is this a known bug or is there something I'm not aware of?