Steps to repro:

1. Open Unity;
2. Create new project;
3. Import attached package;
4. Open 'testBillboards' scene;
5. Select Terrain gameobject in the Hierarchy view;
6. Go to 'Place Trees' tab in the Inspector;
7. Click 'Edit Trees' -> Add Tree button;
8. Add 'Broadleaf_Desktop' tree;
9. Click 'Mass Place Trees' button;
10. Input any number bigger than 10k to 'Number of Trees' field (e.g. '20000');
11. Click 'Place' button;
12. Move the viewpoint away form terrain.

Actual result:
Tree billboards are cut (see attached actual.png screenshot);

Expected result:
Tree billboards should not be cut after mass placing trees (see attached expected.png screenshot).

Notes:
- Reproduces on 5.5.0b5;
- Reproduces on Windows 10 and OSX 10.11.6;
- Reproduces in both Scene and Game Views;
- Reproduces in both Editor and Standalone build;
- Also reproduces for tree creator prefabs, having billboards;
- Seems not to repro for <10000 billboards.