Some times (very rare), Windows Hello fails to recognize me, so I swipe up to enter the 4-digit code, but instead, it unlocks directly, without asking for the code. Has this happened to you? By the way, I am running W10M in a Lumia 950.
Apparently, it is the require Pin every 15 minutes option that is causing this issue, for what I've read in another post. Will give it a try to see if it gets fixed.