Unable to paint holes smoothly on Terrain by dragging the mouse.

Steps to repro:
1. Create a new project > Create a new Scene.
2. GameObject > 3D > Terrain.
3. Select Paint Terrain Tab > Select Paint Holes.
4. Click on Terrain and drag the mouse.

Actual Result:
Unable to paint holes by dragging the mouse on Terrain.

Expected Result:
Able to paint holes by dragging the mouse.

Reproducible in:
2020.1.0b1, 2020.1.0a25.

Working fine in:
2020.2.0a2, 2020.1.0a24

Environment:
Occurring on Windows 10 & Mac 10.14.