Killing a wandering trader will leave their trader llamas leashed even though the trader has died. Killing the trader will not drop the 2 leads on the llamas. Before 1.12 beta, the trader would drop leads when killed, now it's only the llamas who drop those items.

To Reproduce
1. Spawn wandering trader
2. Kill the trader
3. Look at the leash on the llamas

You will notice that the trader dies and the leash for some reason didn't drop as an item after the trader dies, the leash is attached to the llamas. Also you cannot ride the wandering llamas as they are still "leashed to the trader", you have to get a lead and leash the llamas in order for you to ride them.