I have tried taking the bullets from fully loaded magazines by emptying them in the "Vicinity" tab (so I could then drag them into my ammo box), but if you have a full inventory, the bullets are not loaded onto your body.
So they empty into nothingness and are deleted.
Your inventory is full
you find a magazine and hit tab to see the magazine in the Vicinity
right click the magazine to empty the bullets
ammo disappears and magazine is empty.
expected results: bullets empty out of magazine and appear in "vicinity" tab.
This also happens with ammo packets. If you empty an ammo packet in vicinity with a full inventory, the ammo disappears.
Actually it happened to me even though my inventory had plenty of empty space: I just emptied magazine in vicinity and all ammunition disappeared.