When killing an entity that is in a different dimension than the player, the game does not recognize that a player kill took place. The statistic for the respective mob does not update.

I confirmed with this advancement, it triggers if the player and entity are in the same dimension but not if they are in different dimensions.

{
    "criteria": {
        "test": {
            "trigger": "minecraft:player_killed_entity",
            "conditions": {}
        }
    }
}