Reproduction steps:
1. Open the user's attached project "PropCleaningBug.zip"
2. In the Project window select Assets/New Custom Scriptable Object
3. In the Inspector window select the first Scriptable Object's Prefab property
4. Press Shift + Del and observe the field becoming "None" as expected
5. Undo and select the second Scriptable Object's Prefab property
6. Press Shift + Del

Expected result: The Prefab's transform property becomes "None" when pressing Shift+Del
Actual result: The Prefab's transform doesn't become "None" when pressing Shift+Del

Reproducible with: 2019.4.14f1, 2020.1.11f1, 2020.2.0b10, 2021.1.0a4
Not reproducible with: 2018.4.28f1

Notes:
- The issue doesn't reproduce on Mac
- The issue won't reproduce if only the Del or Backspace button is pressed
- The issue reproduces not only with Prefabs, but Materials as well