Repro steps:
1) Open the project I attached
2) Open BugScene2
3) Notice two planes on the left with Static written on it in the Scene view
4) Notice these planes have almost same shaders (only pragma noForwardAdd is the difference)
5) Change the intensity of Directional light in various ways - notice that the second plane from the left is always a little less bright(look at the screenshots)

Expected behaviour: The brightness is the same, since there is only one directional light in the scene

Reproduced in: 5.4.0b4; 5.3.2p1; 5.2.3p3