The bug

If you wear unenchanted armor, the armor points don’t reduce damage when standing on a campfire, even though they do reduce damage from magma blocks. This causes zombies to die a bit quicker from campfires than from magma blocks, because they have 2 armor points by default.

How to reproduce:

1. Wear unenchanted armor, I recommend netherite armor
2. Stand on a magma block
3. Stand on a campfire
4. Compare the damage of the two blocks
   The camfire deals more damage, because the armor reduced the magma block damage, but not the campfire damage.
   Since they deal the same amount of damage without wearing armor, they also should deal the same amount of damage when wearing armor.