Steps to reproduce:

1. Open attached project
2. Opn 'Main' scene
3. Inspect the 'Mission' prefab instance in the Hierarchy
4. Notice the 'Mission Duration' value is overriding the prefab value and set to 300
5. Open 'Mission.cs' script
6. Comment out the declaration of "MissionDuration"
7. Uncomment the declaration of "missionDuration"
8. Go back to Unity
9. Inspect the 'Mission' instance in the scene, notice the "Mission Duration" field is no longer using bold text, indicating it is no longer overriding the prefab value (though it does show the correct value of 300 because that's what's currently in memory)
10. Reopen the scene (doesn't matter if you save or not)
11. Inspect the 'Mission' instance in the scene
12. Notice the "Mission Duration" field has been reset back to the prefab value of 240 and is no longer marked as overriding the prefab

Note: Same behaviour on simple objects (not prefab instances) works as expected.