Unsure if this is intended behaviour or not, but setting "protection" for a custom damage type in a Mob's config doesn't seem to work. It works as intended when applied to a player for example through a modifier, but not on mobs if applied in their config or a modifier added to them. "Protection" can still be set on mobs for vanilla damage types though, and this seems to work normally.

To test I did the following:

• Define a custom damage type in damage_types.yml
• Create a modifier and a mob both with protection: 0.5 for this damage type
• Create a simple damage spell that uses the defined damage type
• Use /mage debug 10 to see damage applied
• Cast the spell on the mob and on a player/self who has the modifier
• See the debug return damage dealt was x1.0 for the mob, but x0.5 for the player