When executing "/effect @e instant_health" and on undead mobs, they will be healed by it rather than being damaged. "/Effect @e instant_damage" doesn't damage them either but instead heals them just like instant health. Splash potions and lingering potions still damage and heal just how they always have.

Because of this, undead mobs become virtually invulnerable to a wide variety of custom attacks. Some can be solved with other methods, but most are far inferior to using /effect.

How to recreate

Step 1: Place a command block. Any type works, but repeating is preferable.

Step 2: Type "Effect @e instant_health". Variables such as selector arguments, duration, and level do not affect the product.

Step 3: Run it.

Step 4: Place a zombie or any undead mob burnt by sunlight. If they do not die after sufficient time, then they are being healed.

The same should work with instant damage. Though, that is to be expected.