The bug

Tipped arrows which have an effect that damages the hit mob do not apply their effect if the damage from the arrow hit is greater than the damage the potion would deal.

How to reproduce

With a bow

1. Summon a zombie with a health of 10

   /summon zombie ~ ~ ~ {Health:10f}
   

2. Shoot with a completely drawn back bow a tipped arrow of healing (level I) at it
   → The zombie is not dead despite the fact that the arrow dealt at least 6 points of damage and the potion effect should have dealt 6 points of damage as well

With a summoned arrow

1. Summon a zombie with a health of 10

   /summon zombie ~ ~ ~ {Health:10f}
   

2. Use the following command; the arrow deals with this motion 6 points of damage and the potion effect has the same amplifier as the healing tipped arrow has

   /execute @e[type=zombie] ~ ~ ~ /summon arrow ~ ~2.5 ~ {Motion:[0d,-1d,0d],damage:6d,CustomPotionEffects:[{Id:6b,Duration:1,Amplifier:0b}]}