Steps to reproduce:
1. Open the attached user's project "3509946.zip"
2. Open the "Assets/Repro/Repro.unity" Scene
3. Make sure "Use new Hierarchy window" and "Default Prefab Mode" is set to "in Context" are enabled in Preferences -> General (when testing on 6000.6.0a6)
4. In the Hierarchy, open "Canvas/OuterInstance" in Prefab Mode in Context (the arrow pointing to the right)
5. In the Outer Prefab stage, open "MidInstance" in Prefab Mode in Context
6. In the Mid Prefab stage, open "LeafInstance" in Prefab Mode in Context
7. Outside Unity, edit "Assets/Repro/Mid.prefab" in a text editor (e.g. add a trailing newline) and save it
8. Return focus to the Editor and wait for the reimport to finish
9. Click the back arrow in the Hierarchy stage header to return to the Mid Prefab stage

Actual result: The Hierarchy only shows a single "Preview" node with no children
Expected result: The Hierarchy shows the reloaded Mid Prefab contents ("Prefab Mode in Context" > Mid > MidChild, LeafInstance > LeafChild)

Reproducible with: 6000.6.0a6 (5c9528587bc8), 6000.6.3f1, 6000.7.0b3, 7000.0.0a1
Not reproducible with: 6000.3.25f1, 6000.6.0a5 (e220e6931c38)
Could not test with: 6000.0.84f1 due to HierarchyV2 not being present

Testing Environment: Windows 11
Not reproducible on: no other environment tested

Notes:
-Project needs to be extracted again to reproduce the issue as the prefab state gets cached (reimporting does not fix it)
-Possibly related to UUM-147870 (preview node reappears in Prefab Mode after layout reload)