Steps to repro:

1. Open Unity;
2. Create new project;
3. Import attached ‘DetailMeshBug’ package;
4. Double click on ‘Terrain’ gameobject in the Hierarchy;
5. Go to ‘Paint Details’ tab of terrain’s component;

6. Notice that there is grass texture along with ‘Cube’ detail mesh (without material’s texture) added;
7. Select ‘Grass' texture in Inspector;
8. Paint ‘Grass' texture on terrain in Scene view;
9. Select ‘Cube’ detail mesh in Inspector;
10. Try to paint ‘Cube’ detail mesh on terrain.

Actual result:
Nothing happens (see attached gif video). When grass texture is added, detail mesh could not be painted on terrain. Inconsistent behaviour within product, because when we have only one detail mesh added (without grass texture) - it’s painted successfully.

Expected result:
User should always be able to paint detail mesh on terrain, even with grass texture added.

Workaround is either to remove grass texture or to add texture to detail mesh’s material and save scene.

Notes:
- Reproducible in: 5.5.0f2, 5.4.0b8, 5.3.3p2, 5.3.3f1, 5.3.2f1;
- Reproducible on both OSX 10.11.3 and Windows 10;
- Interesting behaviour happens, when you have only one cube detail mesh painted on terrain and then add grass texture - detail mesh disappears in this case.