Wither does not regenerate health

DEVICE: Samsung Galaxy Tab A

The Wither does not regenerate health like it is supposed to when it is hurt.

STEPS TO REPRODUCE:
1. Spawn a wither.
2. Hurt it and pay attention to the health bar.
3. It does not regenerate like it should. I believe it should be regenerating a half heart every 20 ticks.

EDIT: It is not supposed to regenerate once below half health but it still doesn't regenerate before then (Hence the bug.)