The Bug

Using the criteria

"criteria": {
    "water_fight": {
      "trigger": "minecraft:player_killed_entity",
	  "conditions": {
		"killing_blow":{
			"direct_entity": {
				"type": "minecraft:potion"
			}
		}
      }
    }
  },

one would expect that any potion would work, as long as it actually kills an entity, but although this code allows the advancement to be triggered when a damage or healing potion is used to kill an entity, in the case of killing a blaze with a splash water potion, it fails to trigger.

This is odd, because if you take the "killing_blow" condition away, it triggers using splash water potions just fine. It even considers it to be "is_magic" correctly. It's only if you specify that it's a potion that it fails.