Steps to repro:

1. Open attached project and 't1' scene;
2. Observe terrain's rendering.

Expected result:
Terrain is rendered with zero smoothness value.

Actual result:
Terrain is rendered completely specular (with maximum smoothness value). See attached video and screenshots.

Regression introduced in Unity 5.6.0b3.

Workaround is to use textures with .jpg, .tif or .psd format as terrain's basemap.

Notes:
- Reproduces in 2017.1.0b1, 5.6.0p1, 5.6.0f1 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 with .png or .gif textures with alpha channel used as basemap;
- Doesn't repro when .jpg, .tif, .psd textures are used as basemap;
- Reproduces in both Editor and Standalone Player.