Steps to reproduce:

1. Open Unity;
2. Open attached project;
3. Open 'test.unity' scene;

4. Notice that there is a terrain with painted simple 2d grass texture (default particle texture);

5. Select Terrain gameobject in the Hierarchy view;
6. In the Terrain component of the Inspector click on 'Paint Details' tab;
7. Click Edit Details --> Edit Detail Mesh;
8. Select any fbx mesh (in my case, 'RockMesh.fbx');
9. In the opened 'Add Detail Mesh' window click "Add" button;
10. Observe the grass texture on the scene.

Actual result:
Grass texture gets corrupted (see attached gif video and actual.png screenshot).

Expected result:
Grass texture should not be corrupted after adding detail mesh as a brush to the terrain component (see attached expected.png screenshot).

Notes:
- This issue reproduces only in case of having simple 2d texture as grass texture and fbx mesh as detail mesh in the Terrain component;
- This issue reproduces on both OSX (10.11.1) and Windows (10);
- Reproduces on 5.4.0a5, 5.4.0a4, 5.3.0f4, 5.2.0f3, 5.1.3.f1.