I got an S with a champion that i dont have Chest with, and i didnt get chest... Rito pls?

I started a game of League of Legends with 4 chests available and picked a champ i didnt have chest on ( varus ) and i carried and got clean S, and after the game ended i expected to get a chest but Rito games spagheti code said different. Can someone tell me why did this happen, or if it happend to anyone else, or if it is a patch bug that they will fix or nah. Thanks.