The "best" explanation would really be an issue with the subdial, at least it wouldn't affect the movement. Of course, that would mean replacing the dial. I'd probably try to explain this issue to an authorized watchmaker, just to get a quick feeling if you have a chance (via phone, calling an AD with a service center?). But then I'm pretty sure you'll have to turn the watch in anyway. I'd definitely get the problem fixed under warranty, and avoid possible complications if it concerns the movement. Until then, you may even want to stop playing with the chrono ;-) I'm sure in the end -- with some patience -- it will go well ;-)