Although MC-82046 is marked as 'Works As Intended', something feels incorrect about this verdict. In particular, withers are damaged by splash potions of healing and healed by splash potions of harming, but are unable to receive the same effect through commands.

How To Reproduce:
1. Summon a wither.
2. Get a Splash Potion of Healing, and throw it onto the wither. It will be damaged.
3. Type this command in chat: 

/effect give @e[type=minecraft:wither] minecraft:instant_health 

4. You will receive an error message stating that the target is immune to effects.
5. The previous 4 steps can be repeated using a Splash Potion of Harming and the 'instant_damage' effect.

Expected Behaviour:
Even though withers are intentionally immune to the majority of effects, it is clear that they are supposed to to be damaged by splash potions of healing, and healed by splash potions of harming, much like other undead mobs. Therefore, these two effects should also be applicable through commands, which is currently not the case.

This bug does not apply to ender dragons, who are unaffected by splash potions and therefore should not be affected by the 'effect' command, either.