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Stop Using Your Face or Thumb to Unlock Your Phone
(gizmodo.com)
This is a most excellent place for technology news and articles.
Jesus christ this explains why occasionally I'll pull my phone out of my pocket and it forces me to input the pin rather than the thumb print. It's just one of those mildly annoying things that you wonder about but don't think about enough to search for the answer.
Your situation is more likely to be caused by Android's system to make sure you don't forget your pin. It has a number of unlocks (and a length of time) before it forces you to do the code.