Reproduction steps:
1. Open attached project "NullRef.zip"
2. Open Assets/SampleAsset.asset.meta in a text editor, modify the last character of it's GUID and save the file
3. In Project window, select Assets/GameObject
4. In Inspector window, observe SampleScript component's SA field ("Missing")
5. In Project window, open Assets/SampleScript.cs
6. In Script Editor, add a new public GameObject reference and save the changes
7. In Project window, select Assets/GameObject
8. In Inspector window, observe SampleScript component's SA field ("None")
9. Undo the changes made in Assets/SampleAsset.asset.meta with text editor and save the changes
10. In Inspector window, observe SampleScript component's SA field ("None")
11. In script editor, remove the new field from Assets/SampleScript.cs and save the changes
12. In Inspector window, observe SampleScript component's SA field (reference becomes valid)
13. In script editor, add the field again to Assets/SampleScript.cs again and save the changes
14. In Inspector window, observe SampleScript component's SA field ("None")
15. In Project window, open Assets/GameObject.prefab
16. In Inspector window, observe SampleScript component's SA field (valid when in Prefab mode)

Expected result: The reference to SampleAsset.asset in GameObject.prefab should remain valid before and after steps 11-16
Actual result: The reference to SampleAsset.asset in GameObject.prefab is valid after removing new field and in Prefab mode

Reproducible with: 2019.4.24f1, 2020.3.4f1, 2021.1.2f1, 2021.2.0a13
Could not test with: 2018.4.34f1 (console errors break project)