It seems that the ItemStack is reduced to 0 but not actually removed, causing the item to become phantom. It can be dropped on the ground, repeatedly, and you will not be able to pick it up again because it is 0 items. I can't get rid of this phantom ink sack. If you try to put it in a chest, it duplicates instead.
Edit: Phew, it seems the phantom ink sacks can be absorbed into stack of one or more ink sacks (by dropping them nearby) to remove them from the world, and then dropped normally after being absorbed into a stack to remove them from your inventory. Scary.
JTE
It seems that the ItemStack is reduced to 0 but not actually removed, causing the item to become phantom. It can be dropped on the ground, repeatedly, and you will not be able to pick it up again because it is 0 items. I can't get rid of this phantom ink sack. If you try to put it in a chest, it duplicates instead.
Edit: Phew, it seems the phantom ink sacks can be absorbed into stack of one or more ink sacks (by dropping them nearby) to remove them from the world, and then dropped normally after being absorbed into a stack to remove them from your inventory. Scary.