i dont know if its a bug or not...

I bought about an hour ago a mystery champion with BE and it gave me Lissandra . I played a game with her (ranked flex) i did really well , (13/6/17 KDA) and got an s grade. Now , I'm a guy that plays his main every game so i got 19 keys and i wanted to get a chest just for the fun of it, anyways, I got the s grade on my game, I had 4 available chests and i didn't get the chest I really wanted. That's the "BUG" I guess ... just wanted to share it in case its an actual bug . Anyways thank you for your time and best of luck in the fields of justice!!!