Automatic change of digital screen lock – why?

my daughter uses a four-digit number as her phone's screen lock (Xiaomi Redmi 3S), but one day the phone wrote "incorrect". She has not changed it so that she can not forget it. It happened three more times. In two numbers, she has unlocked it via a GMail account associated with her phone, but in the latter case, she is not allowed to do so.
1) Is there a way to unlock by GMail account again?
2) What do we prevent to do to avoid self-locking in the future?