[GAMEPLAY] No chest?

Just played a game with a champ i have gotten an s on this season, but at a time where i wasnt eligeble for a chest (renekton). He is mastery rank 5. Got another s, should have gotten a chest, as i was eligeble for a chest at this time, but instead got the blurb about a mastery rank 6 token. Ok, fair enough, i can get another s. Went to check him in my profile, and now he is flagged as not chest eligible. I just got screwed out of a chest on a champion i can actually get a chest on, because of mastery rank 6.