Instant Damage arrows (and Instant Healing for undead mobs) don't work properly.
To check it properly, I did some tests. I used villagers since they are not undead mobs and you can give them armor easily with a dispenser.
I did 6 tests (each of them several times): Villager with Full Protection 4 vs Power V normal (7 shots), Villager with Full Protection 4 vs Power V instant damage (7 shots), Villager with full diamond vs Power V normal (3 shots), Villager with full diamond vs Power V instant damage (3 shots), naked villager vs Power V normal (1-2 shots), naked villager vs Power V normal (1-2 shots).
The only time when I would notice a difference is when I didn't fully charge the bow, instant damage arrows always win in that case. Even weirder: when using instant damage arrows charge time didn't matter. I would kill the full Protection IV villager both in 7 fully charged shots, or 7 not charged shots.
I'm surprised no one noticed this issue with a feature released so much time ago. I can do more tests if needed, but I think it's not. It pretty much makes those arrows pointless so I hope it's fixed soon.
Dupe of MC-107856