Look at the pictures attached, I've circled the cut-off lever end in red. After using the forward assist and before firing the gun crack it open and look at this lever end, with the gun open and the battery out put the gun to Semi and squeeze the trigger, does the white plastic piece go over or under the metal tab on the cut-off lever? It may be that the gears are pushing on this after they reverse and lifting it just enough to be switching it to full auto for the first shot.
How it might look
How it should look
I'm still trying to figure out how to correct this if that is the problem, but if it is we've at least got it narrowed down.