Steps to repro:

1. Open Unity;
2. Open attached project and ‘testAPI' scene;
3. Select Terrain gameobject;
4. Input ‘0’ to Scale field;
5. Launch the scene;

6. Notice that billboard disappeared - EXPECTED;
7. Input ‘2’ to Scale field in Inspector;

8. Scroll closer for upper LOD2 to appear - notice that it's scale has changed to 2, while billboard is two times smaller - its value is 1. See attached gif video.

Expected result:
Billboard scale should be consistent with tree LOD scale when calling it in Play mode.

Notes:
- Reproducible in 5.6.0a2, 5.5.0b11, 5.4.2p2;
- Reproducible on both OSX 10.12.1 and Windows 10;
- Happens only when calling heightScale in Update() method, not happening when calling it in Start();
- Happens on both Editor and Standalone build.