As far as I understand FaceID makes a "map" of your face using a series of sensors (including IR) that is then compared against with when unlocking the phone or using a FaceID compatible login in an app. The beard/facial hair is part of that map, and the way Apple designed FaceID is to learn details that slightly change over time like a beard growing. It does (from experience) fail if the change is drastic, such as shaving a long beard or being in direct sunlight when you haven't been in a long time, but you can then enter the PIN/password and it will learn the "new map data" of your face and will work during subsequent tries. I would guess it does somehow combine old and new data when you enter a PIN and update the map, as it doesn't get tripped up if you, for example, unlock your phone in direct sunlight and then unlock in lower light shortly after, but I can't be certain.