To reproduce:
1. Open attached project;
2. Open "Test scene" scene;
3. Check the Game view.

Expected result: in the first pass Stencil Buffer value should change to 0 using WriteMask set to 0.
Actual result: WriteMask is ignored when writing to the Stencil Buffer.

Reproducible on versions: 5.4.4p1, 5.5.1f1, 5.6.0b7.

Note: Shader "StencilTestShader" has 3 passes: in the first pass the value of Stencil Buffer has to be changed to 0 using WriteMask and fills the pixels with a red color. The second pass checks if Stencil Buffer value is 0 and fills pixels with a green color. The third pass checks if the value of Stencil Buffer is still set to 255 and fills pixels with blue color.