Wither does not use all 3 heads to attack

When the Wither has multiple targets in its detection range, it is supposed to be able to utilze all three heads to attack up to three targets at once. However, it currently does not do that. It only uses its main head.

STEPS TO REPRODUCE:

1. Spawn in a Wither.

2. Spawn in as many mobs that it will target as you want.

3. Watch as it only attacks one mob at a time.