Haven't gotten a key fragment in 75 games...

Yeah, the above are correct. And I would guess the drop is extreme so that it's basically impossible to amass keys at a rate higher than your chests/month. It feels pretty bad, frankly.

It is "random" in case you dont understand what "random" means it can theoretically happen every game or it can happen never so.......intended.

"The drop rate for key fragments is a curve, not a line. With each fragment you earn, the chance to earn your next one goes down slightly. We wanted to give players who didn't play as frequently a good drop rate for keys, without making it feel like farming was mandatory if you wanted keys."