So while I was playing the new rengar today I kept on getting passive stacks without getting the kill. I had 2 kills but 4 stacks? I did help with the kills but didn't get the kill. So I'm not sure what happened but has this happened to anyone else?
That likely is not a bug, based on what you have written. Bonetooth Necklace does not use the same rules for Kills and Assists as other elements in League. Rengar will still get the trophy if he was fighting the champion at that same time, regardless of who is assigned the credit on the scoreboard.
