How to reproduce:
1. Open Unity
2. Create a new project
3. Create a new scene
4. Save scene and close Unity
5. Reopen created project
7. Make changes to 'Main Camera' (Position, Rotation, Scaling)
8. Check DynamicsManager.asset and Physics2DSettings.asset modification time stamps

Expected result: DynamicsManager.asset and Physics2DSettings.asset are not modified as no changes to physics options were made
Actual result: DynamicsManager.asset and Physics2DSettings.asset get overridden although no changes were made

Note: comparing files before and after saving revealed that no data changes were made. The file was just cleared and flushed with the same information.

Regression first introduced in - 2017.2.0b7
Reproducible with - 2017.2.0b7, 2017.2.0b8, 2017.2.0b10, 2017.2.0f2, 2017.3.0a4, 2017.3.0b4, 2018.1.0a1
Not reproducible with - 5.6.3p4, 2017.1.0a1, 2017.1.0f1, 2017.1.0p5, 2017.1.1p2, 2017.1.1p4, 2017.1.2f1, 2017.2.0b4, 2017.2.0b6, 2017.3.0a3