I have already played one game this training week, but it appears in my weekly stats page that I have not. Is this a known bug?
That was during your game. I'm not sure it's a bug at all. The intention is to not give away how many minutes a player has played in a game that is in progress from the user's perspective. It's possible the only way the BBs could do that right now is to not show any minutes.
If the page is just saying that no-one has played, and not a message saying that minutes aren't shown while games are in progress, then that's probably worth calling a bug.