The bug

Since the Wither's "armor effect" is translucent, entities behind it will sometimes disappear (does not render).

How to reproduce

1. Run the following command:

    /summon wither ~ ~ ~ {NoAI:1b,Health:150.0f} 

2. Spawn a mob behind the wither
3. Observe
   → The mob disappears behind the wither's armor effect.