Steps to Repo:
1. Hierarchy > 3D Object > terrain
2. Terrain > raise and lower heightmap > select any brush > raise terrain height
3. Hold shift and click to lower heightmap > observe artifacts on the brush
4. Terrain > Paint smooth height >observe artifacts on the brush

Actual Result:
Artifacts are visible on the brush

Expected Result:
No artifacts are visible on the brush

Frequency:
Always

Environment:
2017.3.0b11
Occurring on windows and Mac

Note:
Once this starts to occur, artifacts on brush are visible when painting /raise/lower, height, texture and smooth on heightmap

Reproducible with:
2018.1.0a6, 2017.3.0b11, 2017.2.f3, 2017.12f1, 5.6.3fc