custom damage types in data packs do not update on /reload

While in a world with a datapack containing a custom damage type, changing it in some way (like the death "message_id" value for example) will not update it in game after /reload

Steps to reproduce:

1. Create a datapack in a singleplayer world, create a json file for a custom damage type called "custom_damage" within the namespace "test_pack".

2. Inside custom_damage.json, paste this code: 

   {    
   "exhaustion": 0.1,
   "message_id": "mob",    
   "scaling": "when_caused_by_living_non_player"
   }

1. Join the singleplayer world

2. Go back to the custom_damage.json file and replace the "mob" value in message_id to "generic"

3. Return to the game, type /reload

4. type /damage @p 1000 test_pack:custom_damage

Observed result:

Death message says "%1$s was slain by %2$s"

Expected Result:

Death message says "{player} died"