I've compiled my app using 108 Beta 11 and it does in fact now call up the requested leaderboard.
However, sometimes it calls up the default leaderboard no matter which one I'm calling, with only the name "Leaderboard" instead of it's proper name.
It seems to call up the default board at random, with no real discernible pattern to it, so that's confusing. If I just open and close Game Center repeatedly, it will just randomly throw in a call to the default leaderboard, with just the generic name "Leaderboard".
Very strange indeed, but is it AppGameKit or Game Center?