Steps to repro:

1. Open attached project and 't1' scene'
2. Select 'Terrain' gameobject in the Hierarchy;
3. Open 'Place Trees' tab in the Terrain Inspector;
4. Input '100' into Brush Size and Tree Density fields;
5. Select any brush in the pallette;
6. Click on terrain once.

Expected result:
Multiple tree instances are randomly painted on terrain. See attached expected.gif video.

Actual result:
Only one tree instance is painted at the center of terrain's brush. See attached actual.gif video.
Regression introduced in 2018.1.0a6.

Notes:
- Reproducible on 2018.1.0a6;
- Not reproducible in 2018.1.0a5, 2017.3.0b11, 2017.1.0p4;
- Reproducible on both Windows 10 and OSX 10.13.1.