Hierarchy v2: Failed to preserve expanded/collapsed states when switching between scene and prefab context mode

Steps to reproduce:

1. Create a prefab with multiple children and place it under a game object
2. Click > to enter prefab context mode 

note: if not repro, try reparent the same prefab to a different node and open in context mode

Actual results: all children are collapsed and expanded states can't be preserved when switch back and forth between scene and prefab context mode

Expected results(Edited): 

• When opening the first time:
  • In Isolation: Expand root objects
  • In Prefab Mode in Context: Expand "Prefab Mode in Context" node + prefab root objects.
• When opening after the first time:
  • In Isolation: Restore state as it is.
  • In Prefab Mode in Context: Expand "Prefab Mode in Context" node, restore the rest as it is.

Reproducible with versions: 6000.6.0a6

Not reproducible with versions: 6000.6.0a5

Can’t test with versions: 

Tested on (OS): mac/windows

Notes:

• Isolation mode seems to be fine
• If it doesn't repro, try to reparent the prefab to another node and then go in context mode