I've noticed ammo boxes will sometimes disappear completely from the inventory when logging in. I have no idea how to reproduce it as it only happens once in a while, but it's definitely a bug that has happened to me on several occasions. Just a couple of hours ago I had been filling up my ammo box with piles of 60 rounds, logged out, joined another server and my ammo box disappeared.

It does not disappear whilst being logged into the server, only when joining a server.