Champion Highest Grade not Displaying Properly in Client

I have played a number of games on all of my owned champions, but for some reason if I get an S rank on a champion that I did not receive a chest for, and have no available chests to receive, the "Champion Highest Grade" stat does not update for that champion. For example, I have received S ranks but no chests for Malphite and Nidalee, but the game only shows my highest grade that I got AFTER getting S, but not S. So for Nidalee it shows my best rank as A+, and for Malphite apparently my best is only B. And for Tristana, which I've played at least 5 games in normals with, has not received a recorded grade at all. Her highest grade is just "N/A". I have yet to get S rank on Tristana.