When the player picks up a stray's slowness arrow (mob drop) it enters the players inventory, however any attempt at collecting other slowness arrows do not work, the player appears to pick up the item but it is not in the inventory afterwards leaving only the initial slowness arrow.

Thinking more about it and observing this bug I realized it may be related to the same bug that affects the crafting of honey blocks and there needing to be space in the visible inventory slots as the arrows were able to stack when in the constantly visible inventory slots while they disappear when in the other inventory slots.