How to reproduce:
1. Open the attached project and set it up as collab with an initial commit
2. Open "TestScene" and expand "Parent" object in Hierarchy Window
3. From Project Window delete "Child" prefab
4. In Project Window right-click "Parent" prefab and select "Reimport", check changes in Hierarchy Window
5. From Collab Window revert deletion of the prefab, check changes in Hierarchy Window

Expected result: After reverting deletion all object instances in Hierarchy are valid and linked to prefabs.
Actual result: After reverting the deletion of the "Child" Prefab, the reference inside the "Parent" object instance remains invalid.

Notes: Does not happen if "Reimport" is not executed. Another Reimport after restoring fixes it back.

Reproducible with: 2018.4.0f14
Not reproducible with: 2019.2.16f1, 2019.3.0f3, 2020.1.0a17
Could not test with: 2017.4 - nested Prefabs not supported