Questions about Locking Doors

I just found out that when I open the tailgate using the keyfob, it doesn't lock back when I close it. It doesn't matter whether the doors were locked using the keyfob or manual lock button. Is it supposed to be this way? I'll check to see if the security system still stays armed after opening the tailgate. In my other MB car, the trunk will lock back and the security system reactivated.