Wither bullets sometimes fail to deal damage

Steps to Reproduce:

1. Summon a command block.
2. Set the command block to purple and type: /effect @e clear
3. Changed from redstone need to always active.
4. Summon the wither and the golem out. 

Observed Results:
Wither projectiles were unable to inflict damage to iron golems sometimes.

Expected Results:
Wither projectiles should always inflict damage.