Sometimes when you unpack a pack of ammo, or eject the ammo from the gun, the ammo stack that had already been in the inventory goes "0", like it had 0 bullets. Sometimes it happens to a single stack, sometimes to more. It never happened to me to more than two stacks. It doesn't seem to matter how much ammo was in the stach that zeros or the stack that joins the inventory.
The "0" stack cannot be split then or loaded, the ammo is basically useless then, and you can only throw it away. I've lost a lot of ammo that way.
It also seems to happen more often when you have little space in your inventory. Before I thought it only happens when there's no space at all for the ammo to go, or just a single slot left, but it happened to me with 3+ spare slots as well.