Reproduction steps:
1. Open the attached project (GrabPass Bug.zip);
2. Open the Main scene;
3. Observe how the sphere is rendered;
4. Change the sphere's material to WithoutGrab from the project assets;
5. Observe that the sphere is rendered lighter than before.

WithGrab and WithoutGrab are identical materials, except the shader in WithGrab has the line "GrabPass {}" added before all passes in the first subshader.

Expected results: both materials will work the same.
Actual results: GrabPass affects the shader and changes colors.

Reproduced on: 5.3.5p8, 5.4.3f1, 5.5.0f3, 5.6.0b1.
Reproduced on Windows 10.