Steps to reproduce:
1. Open the attached project "Case_1138011"
2. Navigate to Project -> Assets -> Scenes and open SampleScene
3. In the Hierarchy window select the "Terrain" GameObject
4. In the Inspector window select "Paint Terrain" option
5. Under the "Terrain Layers" select "Grass Texture Paint"
5. Paint the Texture on the Terrain

Expected results: Texture is painted on a Terrain with a selected Brush style
Actual results: Texture is painted jittery in a square shape with any Brush style

Reproducible with: 2018.3.0a6, 2018.3.10f1, 2019.1.0b8, 2019.2.0a9
Not reproducible with: 2017.4.24f1, 2018.3.0a5

Notes: The bug only occurs if the Brush is dragged across the Terrain. There were some changes made to Terrains in Unity version 2018.3.0a4.

Fixed in : 2019.3.0a1