Sorting Group is not correct in prefab mode

[Henry_Xie]

I have a UI prefab. There are some sprite in this prefab, some sprite is under a sorting group, some set sorting layer in SpriteRenderer.
As I wished, the single object with lower layer should be covered by the objects under sorting group with high layer. It did in game mode, but wrong in prefab mode.
That’s a sample prefab, in UI environment. white circle set its order in layer like -1000, green and orange circle is under sorting group , which order in layer is 0. The white circle just covered everything under sorting group, no matter which layer it set.

It will be right if the prefab is not in canvas environment and game mode.

recurrence steps：

1. Create a prefab with canvas
2. Create a empty gameobject named GroupNode.
3. Add sortringGroup to the GroupNode, set order in layer like 0.
4. Add a sprite as child of GroupNode.
5. Add a new SpriteRenderer outside GroupNode, set the order in layer like -1000.