The bug

When you have absorption hearts, they are supposed to absorb all damage, protecting your regular health bar until they are depleted. This works for all damage types except for wither, poison, and magic (i.e. instant damage potions). In these cases, your regular health is depleted, making absorption completely useless. You can even die while still having a full row of absorption hearts.

How to reproduce

1. Eat an enchanted golden apple

2. /effect @p regeneration 0
   /effect @p resistance 0
   /effect @p wither 100 20 

3. Watch as your regular hearts deplete, and you die with 8 absorption hearts unused.