Steps To Reproduce:

1. Create a Project
2. Hierarchy > 3D> Terrain
3. Terrain > Paint Terrain Tab > Set Height
4. Set Height to 600
5. Click on "Flatten.
6. Select a brush and paint height

Expected Result:
At max height, height shouldn't be painted

Observed Result:
Height is created in the opposite direction

Reproducible On: 2019.2.0a2, 2019.1.0b1, 2018.3.0a8

Not Reproducible On: 2018.3.0a7

Environment:
Occurs only on Mac