For testing I placed a WitherBoss spawner in my world in a in a bright room.

What I expected to happen was...:
Nothing.

What actually happened was...:
The Wither health bar appeared on the screen on the sky became darker, even if I'm not looking at the spawner.
This also overwrites the health bar of any existing Withers in the world.

Steps to Reproduce:
1. Place a Wither Spawner in your world in a bright room (use for example TooManyItems, MCedit or NBTedit)
2. Now the Healthbar will appear.
3. Walk or fly away from it and look in the opposite direction of the spawner.
4. Spawn a real Wither and deal some damage to it.
5. The health bar doesn't chance.