Twice on experimental build I've had a partial stack of ammo change to 0 quantity when emptying a full ammo box of the same ammo into my inventory. Once with SKS ammo, once with Magnum ammo. A stack of 4 turned to 0, and when combined with the new stack of 20 from the box stayed at 20 rounds. With SKS it was a stack of 19 rounds that turned to 0. Both times I had other full stacks of the same ammo that were not affected.