Chests

in my game a teammate scored S rank but i didnt get a chest, i didnt get any chest yet so i have still 4 available and i didnt get one on my champ before.

is this a bug or does it only count when i get the S rank myself? or is it only S+ since our viktor only got a S

(from what i understood before, i should get a chest if me OR a teammate gets S-, S or S+ if my champ was eligible and if I have free montly chests open, does the person who gets S need to have a champ thats eligible maybe, but that was actually the case, i own Viktor myself also.)