Are key fragments dropping properly?

I got my four boxes in a day, but I've had 5 key shards drop since the release.. after quite a lot of games. I've noticed pretty much all of my victories are S's, though, so i'm thinking maybe keys won't drop if you've gotten all your boxes and get an s or something? on an unrelated note, my last two s's didn't give me a box even though it says i have one available...