that's a oreo bug..try this->
1. disable smart lock (settings-> security &location->trust agents-> then disable smart lock)
3. enable smart lock from trust agents..
this should do the trick..
I confirm @Kas solution is working for me, I have to do those exact same steps from time to time, i.e. when the face icon is not displayed on lock screen.
i found that solution on an oneplus forum.. so yea it's an oreo bug as it's not specific to a particular device