You didn't have a 23/1/8 game on yasuo in the past month, and you've never come close to 10 cs/min let alone exceeding it.
A couple hours ago you went 23/1/2 with 134 cs in a 24 minute game, and around 3 weeks ago you went 27/2/8 with 167 cs in a 26 minute game, but there weren't any game with the exact score you said or anywhere near the cs in the past month. It's possible you misread some of the numbers in the end-game stats which lead to your confusion about only getting an S-, as getting 10+ cs/minute is the level of pro players and would probably result in an S+.
In any case, if you've managed S grades before, it's just a matter of keeping at it and doing what you did in those games, and/or just hoping for those really stompy matches and csing as best you can. (Keep in mind that objective participation and kill participation, along with warding are also known factors to grades, so don't totally neglect those things while you try to farm.)