Bug with mastery system?

Found an odd bug. I'm shooting for the last S for to get Mastery 7, and I seem to only get S- for some reason(I have 20+ S- ever since I needed only one more, lost count after awhile, could easily be 30+). Just played a game where I went 6-0, killed or helped kill 5 towers and an inhibitor (only ones that fell in game by the way other than bot's first tower), had most farm in game by a decent margin, most damage dealt to champions, etc. Was 90% of the reason the enemy team ff'd at 20 minutes. Other than maybe missing a few less CS (150 at 20 minutes seems decent though), I don't know how I could have done better.

Seems like a coding error or something to me, can we get a fix that allows for S level games to be counted as such?