Reproduction steps:
1. Open the attached project "BaseProject.zip"
2. In the Project window under "Assets/_TerrainAutoUpgrade" open the "TestScene.unity"
3. In the Hierarchy window select the "floor" GameObject
4. In the Inspector window under the Terrain Component select "Paint Terrain"
5. In the Terrain Layers section select the last visible Terrain Layer
6. In the Scene View begin painting the Terrain

Expected result: black artifacts do not appear on the Terrain when painting it
Actual result: black artifacts appear on Terrain when painting it (See attached Terrain.mp4)

Reproduces on: 2019.4.12f1, 2020.1.9f1, 2020.2.0b7, 2021.1.0a2
Could not test on 2018.4 due to the project breaking when downgrading

Notes:
-A workaround is to create the Terrain from scratch
-Issue does not reproduce on a new Project or when the Scene is exported from the attached Project