I summoned a Wither Boss with the summon command and added NoAI and the health bar didn't go down when I hit him. Tried doing the same thing without NoAI and the health bar worked correctly. The Wither died after hitting him enough in both cases. The two first images represent the Wither with the NoAI tag. He is below half health as there is the glow around him to prevent ranged damage. The two last images represent the Wither without the NoAI tag. He is also below half health.