playerkills Scoreboard objective and player killed entity advancement do not track correctly

If there are 3 or more players, the scoreboard objective "custom.playerkills" and the advancement trigger "player killed entity" will give the kill to the first player to hit an entity, rather than the player that actually kills it.

Example:
Player A hits Player B
Player C kills Player B
Player A gets the kill on Player B

Both the advancement and the objective will trigger/add for Player A in this scenario.