Using the /effect command to apply instant health to undead mobs (Zombie, Skeleton, Wither Skeleton, Zombie Pigman and Wither) don't hurt them even if they are doing the hurt animation and sound.
/effect @e[type=!Player,r=20] 6 1000 255 1
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Confirmed.
Splash potion of healing does kill that mobs.

It appears that the level of the potion is the cause here: Setting the Amplifier to below 125 works.

Ah. Thanks. That way duplicate of MC-10755