Wither Bosses Only Summon 3 Wither Skeletons

On hard difficulty, when the wither boss reaches half health it will only summon 3 wither skeletons, where as normally it would summon 4.

How to reproduce:
1. Set difficulty to hard
2. Summon a wither in a contained area
3. Deal damage to it until half health, and you will see it only summons 3 new wither skeletons, not 4 as is normal in the 1.9 update and before.