From what I understand, when supporting, the statistic that mastery ratings take most into account is the assist-to-teamwide kill ratio, or kill participation.
You have a 24 kill participation out of a 45 kill game from your side, which is roughly 53.33% total KP, just barely above the 50% mark, and I think the cut-off for S rank support with less than 7/8 kills is 55%/60% Kill Participation.
It's easier to achieve an S as a damage-dealing support like say Lux or Brand if you top the damage-to-champions chart with only a few kills but only one or two deaths because the damage-to-champions stat tends to be weighted more heavily. If you are playing a utility/low damage support, then you most likely have to incur at least a minimum kill participation if you are not the one collecting the kills.
I'm mostly just speculating but I'm relatively certain that for the particular game you played, Jhin and Kayle must have achieved multiple kills that did not have you as a kill-involvement, and thus your KP tanks. It's a little ridiculous that Riot wouldn't account for the fact that if you as a support smash lane/mid-game and allow your carry(ies) to get fed, that they might go off on their own and continue to get kills while your own KP hemorrhages.