The arrow of harming deals even LESS damage LESS damage then is used to. Now instead of dealing a heart of damage more than the other arrows it deals half a heart of damage.
A suggestion for the arrow of harming would be to make it deal 11 damage or 17 damage, because those are the damage amounts of being hit by a normal arrow then having a splash potion of harming hit you directly. (a splash potion of harming 1 and a splash potion of harming 2