Steps to reproduce:

1. Create new project
2. Import the attached scene (MaskBug.zip) and open it
3. Open 'Build Settings' and switch to Android
4. Check the 'Disable Depth and Stencil' in 'Player Settings/Resolution and Presentation'
5. Reimport the project
6. Notice the mask is not working in the game view (in scene view it works as expected)

Workaround:
Dock the 'Game View' to any other tab in the editor.

Note:
Might be related to Direct3D graphics API as opening with GL Core prevents the issue.

Captured repro/workaround:
http://screencast.com/t/84CaxwV4RF

Reproduced with:
5.1.4f1, 5.2.2f1, 5.3.0f3, 5.4.0a4