I spawned a wither BOSS using the /summon command and I added the tag

because I didn't want it to attack. I started attacking it but the health bar at the top did NOT go down. I spawned a new wither but the same thing happened. I even reloaded the world. The bar was STILL full. So I just attacked it until the bar DID go down. After awhile, the wither armor showed, meaning it was at half-health. The health bar was still at 100%. It's health was updating just fine... but it's health bar at the top just does not change. Another thing I should add: The health bar updated just fine all the "NoAI" tag was set to 0.

Summery: If wither boss has no AI, then the health bar won't update.