Undo action does not work after 'Clear Parent' action is performed when the Prefab object is open in either Context or Isolation mode.
Refer to the attached Video.

Steps to Reproduce:
1. Create a New Project
2. Hierarchy > Create Empty > Create Empty Child
3. Create a Prefab of Parent Object
4. Open Prefab > Select Child > Clear Parent > Undo

Actual Result:
The object is not replaced as a child after 'Undo' is performed

Expected Result:
The object should be replaced as a child on 'Undoing'

Note:
'Clear Parent' feature was introduced in 2021.1.0a1

Reproducible with:
2021.1.0a3, 2021.1.0a1

Environment:
Windows 10 ,macOS 10.15 and Ubuntu 18.04 LTS