We're aware of a communication / UX issue in the way we're communicating to players that their keypool is empty.
Our current implementation can give the impression that it's a bug, but it is actually intended behaviour, we're just not great at letting you know what's going on.
When playing and receiving keys after matches, it is possible to exhaust the key pool. At this point you have to wait for it to regenerate. You can still complete missions to get more keys.
We'll try to improve the way we show that you've exhausted the keys better in an upcoming update, we promise!