Mastery and chest?

Just today, I got an S- with Twitch and was awarded a Twitch Mastery token, but no chest. This has me confused, because I'm almost certain that I once got both a mastery token AND a chest for getting an S on Sivir. Is this a new feature that Riot made so we don't farm chests as much, or is this a bug?

I hope it's a bug, but it wouldn't surprise me if they're just trying to keep people from getting chests this way. The leveling up system, while neat, is much worse for collecting IP/blue essence than the last system, so I don't think they know/care that they're making it harder on us.