[Gameplay] Rengar loses his whole Bonetooth Necklace

I Played Rengar vs Kha Zix and I won the optional Mission "The Hunt is on!". So i got 42% Bonus Attack Damage. But around minute 52 I got killed by Jinx with assistance from Kha Zix and i lost the whole Bonetooth Necklace due to a Bug, I guess. I can´t explain to myself why this Bug happened. Its the first Time i have seen this Bug.