The bug

In combat, using a shield against a mob that has an enchanted sword with the fire aspect enchantment still sets the player on fire. However, the damage from the mob itself is fully blocked while the effect of the enchantment can still damage the player via fire damage.

How to reproduce

1. When summoning a mob with a weapon:

   /summon husk ~ ~1 ~ {HandItems:[{Count:1,id:wooden_sword,tag:{Enchantments:[{id:fire_aspect,lvl:2}]}},{}]}
   

2. Get a shield
3. Go to survival mode hold right-click, and let the mob attack you
   → The player catches on fire.

Code analysis

Code analysis by JingyBreadMan can be found in this comment.