Arrow Deletion With Dispensers

If you create a dispenser and load it with arrows, and have a mob in front of it (the mob is needed for it too work), and then press a button on the dispenser, and then log out after you can see the button go in but before the sound, one arrow will now be gone. I even ran the /kill command which found nothing, confirming the arrow had indeed been deleted. I thought at first that the animation may not be showing but the mobs were still taking damage, but I'm uncertain as to whether that's true.