Inspite of my OnePlus3 being set with a 4 digit passcode and fingerprints anyone can swipe down from notifications on top and back upward and the phone comes open i.e. unlocked. Earlier it was work fine and was secure.
By chance, are you connected to any trusted devices that keep your phone unlocked...? Otherwise that would be a major bug, not supposed to be that way.
Try clearing cache and rebooting. Also try it in safe mode, see if it acts the same way.
I have no idea what scenario can cause this. I cannot wake my phone up (screen double tap) and swipe up or down to get anything other than the the short setting screen where I can turn on/off wifi, etc. Otherwise, my OP3/4.0.2 goes right to the PIN/FPS for authorization.