Thanks, that was indeed a bug. It will be fixed as of the next update.
However, in fixing it, I've decided to address a balancing problem: starting with the next update, you won't be able to reduce your "Trainings to Next Level" stat unless the stat you train is at 4 or above. (This is, of course, to dissuade people from starting with one or two really, really low stats and training them up at low cost to gain lots of levels.)