How to reproduce:
1. Open user-submitted project (PrefabBUG.zip)
2. Inspect Prefab A in Prefab Mode - it's got a missing child Prefab B
3. Open the root project folder and drag prefab B and its meta file into the Assets folder
4. Inspect Prefab A in Prefab Mode again - it now contains child Prefab B
5. See Prefab A Inspector preview outside of the Prefab Mode

Expected result: prefab A is automatically updated to show its re-added child Prefab B
Actual result: prefab A is not automatically updated to show its re-added child Prefab B. Reimporting Prefab A fixes this

Reproducible with: 2018.4.20f1, 2019.2.0a4
Not reproducible with: 2019.2.0a5, 2019.3.7f1, 2020.1.0b4, 2020.2.0a5
Could not test with: 2017.4.38f1(project incompatible)

Notes:
Building an Asset Bundle without reimporting prefab A will not include Prefab B