Its one of those things which I wasn’t happy about with my Pixel4. Who on earth over looked the fact you could use the face unlock without your eyes open! It doesn’t take a lot to think about the abuses including spouses with trust issues.
Finally over the last few days Google rolled out a fix which requires your eyes open if you enable it! Only 5 moths later
It was the first thing I did when I installed the update. Till that point I’ve been enabling lockdown mode when going through sensitive areas like airport security