Steps to repro:

1. Open Unity;
2. Create new project;
3. Extract attached 'terrainTextureBug' package;
4. Open 'test' scene;

5. Notice that there is a scene with 2-layered textures drawn on top of main texture layer;
6. In the 'Assets' folder select 'New Terrain' asset file and click on the arrow at the right side of the icon;
7. Select SplatAlpha0 element of the New Terrain asset;
8. Drag the mipmap slider in the Inspector;

9. Notice that textures drawn on top of the main texture are not rendered on mip-levels lower than mip0 (see attached actual.png screenshot);
10. Scroll out in the scene view.

Actual result:
Terrain textures drawn on top of the main texture are only rendered when being close and very far from terrain (see attached gif video).

Expected results:
All multi-layered terrain texture should be rendered on all mip levels with decreased detail level - both in Scene view and in Inspector's preview (see attached expected.gif video).

Notes:
- Reproducible in 5.4.0f1, 5.5.0a3, 5.3.5p1;
- Reprocucible on both Windows 10 and OSX 10.11.5;
- Also reproducible in 'Gaia' package (https://www.assetstore.unity3d.com/en/#!/content/42618) after creating procedural terrain with multiple textures on it.