Problem: I know you might not consider this as a bug. When you hit a mob or palyer with a tipped arrow it deals damage as regular arrow and also applies the potion effect. However, this is not the case for arrows of harming. If you hit a target with arrow of harming I shot from a fully charged bow, only the arrow deals damage, but if you use arrow of harming II, only the harming effect deals damage, not the arrow.
Reason: Entity cannot take damage from multiple sources at the same time, because taking damage causes entity to become invulnerable for a short time. Instead it only takes the highest damage. Harming I deals 6 damage, fully charged arrow 8-10 and harming II 12. This explains the problems mentioned above. I know this is probably intended, but it is definitely an inconsistent behaviour since other types of tipped arrows deal damage and also apply potion effect. This also maked harming I on arrows almost useless (since arrows do usually more then 6 damage) and with power V bow harming effect on arrows becomes absurd.
Solution: When an entity gets hit by arrow of harming, after taking damage from the firsrt source set it's hurt time to 0, so it will take damage from both arrow and effect. That might sound OP, but I think you can reduca power of the harming effect as it is reduced on a splash potion if it doesn't hit target directly. Again, I know this is more a suggestion, than a bug report, I just think this is something, that has to be fixed.