Method and class names based on 1.11 decompiled using MCP 9.35 rc1
When a mob is shot with a tipped arrow all effects are applied as if they were no instant effects. This causes instant effects with the Amplifier 0 to not be applied because the method net.minecraft.potion.Potion.performEffect(EntityLivingBase, int) can be considered broken for instant effects because it only shifts with the amplifier but does not add 0.5 at the end.
This can be fixed by having the method net.minecraft.entity.projectile.EntityTippedArrow.arrowHit(EntityLivingBase) test if the effect is instant and in that case call the method net.minecraft.potion.Potion.affectEntity(Entity, Entity, EntityLivingBase, int, double) instead. After this change tipped arrows would be affected by
MC-25866 / MC-98123 as well.