To duplicate the error:

Place ammunition in protector case, for example 10 rounds of .380 ammunition. (You might had to have the protector case in your inventory already, idk)

Have protector case in hand and add from your inventory another 10 rounds to the already placed 10 rounds... you'll see that the ammunition in the protector case wont update.

It will however update the amount of ammuniton, when you place the protector case in your inventory and take it in your hands again.

Its not game breaking or anything, just annoying