This has happened to me on Alistar, Ivern, and Mundo this season alone. It sucks, but just gotta earn it again if it doesn't show or register.
You aren't garuenteed a chest on a champ if you get an S- or above. Only one chest can be earned on a champ per season. And I had an S+ on vayne and it only shows my highest as an S, that was weeks ago so I don't care anymore about it
Make sure you had a chest available to earn, and that you haven't already earned a chest with that champ.
Your collection page can sometimes take a little bit to update with season highest grades, so I wouldn't worry about that unless it persists.