To reproduce:

1. Open Unity and create a new project
2. Create Particle system
3. Set Start Rotation between two constants - from 0 to 360
4. Set Render Mode to Mesh and select Quad
5. Set Material with Particles/Additive shader and add round texture
6. Set Y scale to 1.5
7. Enter to the Play Mode

Expected result: In Game window you will see that particles act normal
Actual result: In Game window you will see that particles start rotation behavior is broken - it scales mesh with wrong rotation

Reproduced on Unity 2017.1.0b2, 2017.1.0b8 and 2017.2.0a1
Not reproduced on Unity 2017.1.0b1
Regression on Unity 2017.1.0b2