For those having some problems with Face ID - try turning off
"Require Attention for Face ID". I encountered this when I put my phone into the car holder, and had a few times FaceID failed... then realize when I'm driving my eyes are actually on the road a lot so the phone will not unlock. If you have problem with certain pair of specs or shades, you can also try turning off this settings too.
(Of course you can say with this setting off someone can bring your phone to your face to unlock when you are sleeping.... the same can be said lah even with TouchID... sleep with gloves and ski masks best

)
The auto show of notifications preview will still work with this setting - only when it detects your face, so no one else can see the notifications.
Hope this helps.