Child GameObject of nested prefab is turned into a prefab on completely unpacking the root prefab

Steps to Repro:
1. Open attached or create a new Project
2. Create a nested Prefab AB
3. Add A instance to the hierarchy
4. Right click on A prefab and unpack it
5. Open prefab B and add a child GameObject
6. Return to Hierarchy
7. Undo until Object A is a Prefab again
8. Right click on A again and click Unpack completely
9. Expand A to view all its child Objects

Actual Result:
Child object of prefab B is automatically converted to a prefab

Expected Result:
Objects should not convert to prefab when unpacked completely

Occurring with:
2019.3.0a1, 2019.2.0a14, 2019.1.0f1, 2018.3.8f1, 2018.3.0b5

Environment:
Windows and Mac