The problem

When a player (lets say player A) throws a lingering potion and kills another player (player B), it will say that player B was killed by the item player A was holding at the time of player B's death (if the item has a custom name)

How to recreate (singleplayer)

1. open a new singleplayer creative world
2. give yourself any item, and rename it in an anvil
3. throw the lingering potions of harming
4. hold the renamed item and kill yourself using the lingering area effect clouds
5. the death message will say "Player was killed by Player using <renamed item>

How to recreate (multiplayer)

1. open new creative world and open it to lan
2. join the world on another computer (player B)
3. have player A get lingering potions of harming
4. have player A get any item and rename it using an anvil
5. have player B enter survival mode
6. have player A throw the lingering potions at player B, then quickly switch to the renamed item
7. Player B's death message will say "Player B was killed by Player A using <renamed item>

What is supposed to happen

Death message should say "Player A was killed by Player B using magic"