To reproduce:

1. Open attached project
2. Select test object in the hierarchy
3. In the Inspector, set the M1 age to 10
4. Set M2 age to 20
5. Press M1 = M2 button
6. Set M2 age to 25. See both properties change
7. Set M1 age to 12. See the property reset to 25.

Expected: Changing one property after making both properties equal should change the other property and not reset the first one.
Actual: Changing one property after making both properties equal does not change the other property and resets the changed property.

Reproduced on 5.5.1f1, 5.5.0f3, 5.6.0f2, 2017.1.0a5.