Reproduction steps:
1. Open user attached project "BugTests"
2. In SampleScene, select "TESTING"
3. In the Inspector, toggle between First and Second Reference button
4. Click Rebind Property

Expected Result: Switching between References updates the sub classes.
Actual Result: Sub classes do not update.

Reproducible with: 2019.4.28f1 (the field is frozen), 2020.3.13f1, 2021.1.13f1, 2021.2.0b1 (the field is not frozen but not updated/empty as well)

Notes:
- The propertyFields Freeze after the SerializeReference property they are bound to changes type. Rebinding the field to the property updates and unfreezes it.
- "SerializedProperty has disappeared" error is logged when switching references