How to reproduce:
1. Open the user's attached "Test-Terrain-Holes" project
2. Open the "TerrainTest" Scene
3. In the Hierarchy select the "Terrain" GameObject
4. In the Inspector select the Paint Terrain Tools and Paint Holes tool from the tool drop-down list
5. Paint a hole in the Terrain

Expected results: All the painted textures are rendered
Actual results: The 5th Layer texture stops being rendered (it becomes solid black instead of having dots)

Reproducible with: 10.1.0 (2020.2.0b12), 10.2.2 (2020.2.2f1), 11.0.0 (2021.1.0b3)
Not reproducible with: 4.10.0-preview (2018.4.31f1), 7.5.2 (2019.4.18f1), 8.3.1 (2020.1.17f1), 10.0.0-preview.27 (2020.2.0b5), 11.0.0 (2021.2.0a2)

Notes:
- The issue is not reproducible with Built-in and Universal RP