Earned an S+ but no chest.

So I just finished an amazing game, we absolutely stomped the enemy team and at the end of the game I achieved an S+ rank with heimer for my performance bizarrely I did not receive a chest for this. I have not had any punishments in place (and I never intend to, as flaming etc ruins the game for everyone) and thought this was rather odd. do I need to have lv5 mastery with the champ/be in a premade to receive the chest or is this some sort of bug. any help would be appreciated as I got the last part for a key form that game as well so I was really looking forward to opening it.
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