How to reproduce:
1. Open the user's attached "SerializationTest" project
2. Open the Scene "SampleScene"
3. Select the "TestObject" GameObject
4. In the Inspector observe the "Test Script" "New Name" variable

Expected results: The "New Name" variable's value is "OVERIDDEN NEW VALUE!"
Actual results: The "New Name" variable's value is "OVERIDDEN OLD VALUE!"

Reproducible with:2019.4.16f1, 2020.1.12f1, 2020.1.16f1, 2020.2.0b10, 2020.2.0b14, 2021.1.0a8
Not reproducible with: 2018.4.30f1, 2019.4.15f1, 2020.1.0a1, 2020.1.11f1, 2020.2.0a1, 2020.2.0b9

Notes:
- Steps to reproduce this issue from scratch:
1. Create a Prefab and attach script with "public string oldTest = "A";" line to the Prefab
2. Add the Prefab to the Scene and in the Inspector change the variable's value to "OLD"
3. In the script add "[FormerlySerializedAs("oldTest")]" before the previous added line and change the variable's "oldTest" name to "newTest"
4. In the Inspector change the variable's value to "NEW"
5. Restart the Editor, the value will be "OLD" in the Prefab's that's in the Scene