I don't know what causes the glitch but I've had ammo disappear on a constant basis for me, usually when I eject the remaining bullets from my mag, some of the stacks (usually those that are non-full) just flat out disappear. Sometimes they just have a "0" ammo count to them, and sometimes there's no ammo count (just bullet icon).
- Legacy ID
Just load/unload, combine bullets, it will happen eventually, doesn't take much.
Confirmed. Happens constantly with all varieties of ammunition. It usually occurs when I empty a mag or a box of ammo into my inventory, another stack of the same type will suddenly show as having a null count.