Granted I've only had this happen specifically as Kha'Zix with Yuumi attached vs Quinn, but it totally ruined the game.
On multiple occasions, Quinn one shot me through stealth with absolutely no reveal setup. To confirm this, I watched the replay from her team's perspective. She literally vaults off of thin air and deletes me as if she has vision.
I can't say for sure whether her allies also had vision of me, but their skillshots were strangely accurate for being aimed at an invisible target with 550 movement speed. That could just be lucky, but targeted abilities? So either Quinn is currently abusing a bug that allows her to target Kha'Zix/Yuumi in stealth, or Kha'Zix is just permanently revealed with Yuumi attached and it's so screwed up that spectator doesn't even understand.
I'd love to post the replay, but attempting to record the interaction crashes League (and blue screens my computer apparently). Possibly unrelated but something is very, very wrong here.
Has anyone had a similar experience? Was super excited to get my Season 4 Q damage back and now here I am getting my head knocked off my either a game breaking bug or a pretty destructive exploit.