How to reproduce:
1. Open the attached "TestFor1064275.zip" project
2. Select Particle System in the Hierarchy
3. Make sure that Show Bounds is enabled in the Particle Effect panel
4. Try to change Particle System Shape Y Scale in the Inspector window
5. Notice that the Particle System Bounds stay intact

Expected result: Particle System Bounds change together with the Shape Scale
Actual result: Particle System Bounds ignore the Shape Y Scale

Rectangle Emission Shape was introduced in Unity 2018.1.0a6

Reproducible with: 2018.1.0a6, 2018.1.9f1, 2018.2.2f1, 2018.3.0a6

Note: using other Emission Shapes results in a correct behavior