Reproduction steps:
1. Open the attached project "Unity 2017 3_1_p4 bug test.zip";
2. Open the scene "Test_Wheel_Physics";
3. Observe how the wheel spins at a constant angular velocity;
4. While the scene is running, select the Canvas and increase the "Scale Factor" to 0.5 or greater;
5. Observe how the wheel behaves.

Expected results: Velocity does not change after changing the Scale factor.
Actual results: the wheel starts spinning faster and faster and behaves completely differently.

Reproduced on: 2017.3.1p1, 2017.3.2f1, 2017.4.1f1, 2018.1.0f1.
Not reproduced: 2017.1.3p3, 2017.2.2p3, 2017.3.1f1 and seems to be fixed in all versions of 2018.2.

Tested on Windows and MacOS