I have not been receiving any chests or mastery medals for getting s ranks i have gotten 5 today and no rewards. yes i do have two chests to still unlock and yes the champion i got the s games with were mastery 6. i got an s on hexakill and normals with no rewards not even a key...
For the chests, you can only earn one chest per champion per season, so if you have already earn a chest with the champion(s) that you got an S grade with, then you will not earn another chest with that same champion.
For the mastery tokens, mastery tokens are not awarded when playing Hexakill, or any other rotating game modes that are not on SR.
Also, you say that you were mastery 6 with a champion? If that's the case, and you were playing on SR with that champion, keep in mind that only S and S+ grades will give you the tokens to upgrade to mastery 7. If you get an S-, you will not get a mastery 7 token.
Hopefully this helps.
