Steps to repro:

1. Open Unity;
2. Create new project;
3. Add Terrain gameobject (GameObject -> 3D Object -> Terrain);
4. Go to 'Paint Tree' tab in the Inspeсtor;
5. Hover on terrain in Scene view;
6. Scroll out in the scene view until you see round blue brush circle with rough edges;
7. In the Terrain Component's inspector switch to 'Paint Height' tab;
8. Toggle the brush size - make it smaller;
9. Try to paint height on terrain, use different brushes.

Actual result:
- Sometimes raycasting stops working, it doesn't modify heightmap on click in the Scene view;
- Sometimes the brush rendering is broken - it consists of 2 'nested' circles now, the internal (dark) one is a real brush which changes upon toggling brush size. Otherwise it looks like the brush from 'Paint Tree' tab is drawn on top of 'Paint Height' brush.

See attached gif video and actual.png screenshot.

The bug is completely blocking the work on both my machines in 5.5 builds.

Regression introduced in Version 5.5.0a3.

Notes:
- Reproducible in 5.5.0a6, 5.5.0a5, 5.5.0a3;
- Not reproducible in 5.5.0a2, 5.4.0f3;
- Reproducible on both Windows and OSX;
- The issue reproduces only when creating new project, it doesn't repro when reopening existing project;
- The issue doesn't reproduce everytime and on each machine, however if it occured once, it will be steadily reproducible;
- There might be other specific steps that lead to constant repro of this failure.