Death Messages All messed Up!

Me and my friend were playing a custom made minigame when he encountered a odd glitch using the off-hand feature in 1.9. When he shot me his bow, it showed in the death message that he shot me with his SWORD because that's what he was holding.

1: Get on any world with a friend (or enemy, or whatever)
2: Hold a named item in your hand and a bow in your off-hand
3: Shoot and kill the target and it should say you shot them with whatever item in your hand

I also have a video on this glich on my YT channel: https://youtu.be/BABt7H_rH8w