I frequently (i.e. maybe every 4th or 5th time) have the case that Windows Hello obviously recognizes me (i.e. the message changes from "Looking for you" to "Hello Michael" and the camera light turns off) but then the screen remains locked, i.e. the lock-screen doesn't disappear.
Typically I then have to select "Signin options", choose Pin and enter my PIN to get access.

What is failing here and why is the screen not released after Hello has recognized my face?

This is on a Surface Pro 4 running Windows 10 Pro with all the current patches/updates.