I have this exact problem,  and I think it's a bug. It's also unsafe because it happens a lot while I'm driving. I reach down to open the phone with my fingerprint (something I can do without looking at the phone,  using only my finger),  but the phone wants a pin. I also have this problem with SmartLock enabled. In fact,  I enabled SmartLock with the headset I havr on all day,  thinking this problem would go away,  but it hasn't. I reboot my phone 2 or 3 times a week out of habit,  and this problem can happen 3 to 4 times in a day. I hope this gets cleared up in an update soon. It's really the only annoyance I have with what is otherwise the best phone I've ever had.