Firstly, this is a bug that was previously claimed to be fixed by Darkflow, and to my knowledge, it only appeared on those open-topped vehicles at the time.
This is my loadout for Pz.3N, as you see only Commander and Gunner got MP 40s.
This is the replay ID, just some bot-farm though.
The situation occurred after the Driver was killed by PIAT. As you can see, whether the commander is observing, manipulating AA MG, or even getting off the vehicle for repair, there are always some kills caused by a Bot with MP 40 in the kill-log, and this Bot can only be the Gunner.
Just to remind everyone to pay attention to this possible situation.