Steps to repro:

1. Open attached project;
2. Add Terrain gameobject (GameObject -> 3D Object -> Terrain);
3. Add some height to terrain (via any painting tool);
4. Go to Paint Texture tool;
5. Click Edit Textures - > Add Texture...;
6. Add any texture and press Add;
7. Go to Terrain Settings tab;
8. Decrease Base Map Dist. value.

Actual result:
Base map distortion switch transition is glitchy; it doesn't preserve brightness of terrain's material.
See attached gif video.

Workaround is to either save scene or switch to Gamma color space.

Expected result:
Base map distortion should have consistent brightness value on different distortion levels.

Notes:
- Reproducible in 5.6.0a3, 5.5.0f1, 5.4.3f1;
- Reproducible on OSX 10.12.1 and Windows 10;
- Reproducible only in Linear; Not reproducible in Gamma.