Steps to reproduce:

1. Open attached project
2. Open scene "Test"
3. Select "Particle System" in hierarchy to simulate it
4. Notice that it emits perfect squares
5. Set X value of scale to 2
6. Notice that particles are four times as long as their height (scale is 4, not 2).
7. Set particle "Render mode" to Billboard
8. Notice that particles get shorter and now their scale is correct

This issue only happens with Mesh render mode.

Reproduced with: 5.3.1p2, 5.4.0b1
Regression introduced in: 5.3.0b2