This thing is excellent and I've been using it in a project for a while, but it looks like 2.3.1 of GM2 causes the shader to get messed up if game_restart(); is used after the shader has been activated once anywhere in the game. At that point, it's as if the settings for the shaders are corrupted and the effects don't look right until the game is re-compiled or the executable is run again.
I tried testing it in the demo and it occurs there as well. Not a huge deal, but I'm assuming something's changed within GM to make this happen and I can't seem to figure out how to fix it.