Steps to reproduce:
1. Create a new Project and download "Grassy Valley Terrains" Asset from the Asset store
2. Navigate to Project -> Search and type "scene"
3. Open the scene.unity
4. If the Scene looks empty, scroll backwards with the mouse to increase the scene view
5. In the Hierarchy window expand GameObject "LAND" and select "x0y2" child
6. In the Inspector window Select the "Paint Terrain" option and set it to "Paint Texture"
7. Select "layer_rock_diffsand2_diff"
8. Paint on all the Terrains without releasing the mouse button and notice the different colour on the majority of the Terrains

Expected results: Painted Layer on multiple Terrains looks exactly the same
Actual results: Some of the Terrains changes the colour of the painted Layer

Reproducible with: 2018.3.12f1, 2019.1.0b10, 2019.2.0a11

Fixed in: 2019.3.0a7

Note: The ability to paint Layers continuously on different Terrains appeared in 2018.3.0a11. This issue occurs with an Asset "Grassy Valley Terrains" form the Asset store
https://assetstore.unity.com/packages/3d/environments/landscapes/grassy-valley-terrains-60497