Prerequisites:
You'll need Unity 5.5 and 5.6.

Steps to repro:

1. Create new project in Unity 5.5.;
2. Export attached package;
3. Open 'test' scene;
4. Observe terrain gameobject's surface;
5. Save project and close Unity 5.5;
6. Reopen the project in Unity 5.6;
7. Open 'test' scene;
8. Observe terrain gameobject's surface and compare it to the one observed in 5.5.

Actual result:
In 5.6. terrain's surface is much more specular (see attached screenshots).

Regression introduced in Unity 5.6.0b3 (358ff4ed3952).

Expected result:
Terrain surface rendering should be same after upgrading to 5.6.

Notes:
- Reproduces in 5.6.0b3;
- Doesn't repro in Unity 5.6.0b2, 5.6.0b1, 5.5.0p1;
- Reproduces on Windows 10 and OSX 10.12.1;
- Also specular rendering reproduces when creating new Unity project;
- Reproduces when upgrading project with exactly the same Terrain, Rendering and GI settings;
- Reproduces in both Editor and Standalone Player.