I just figured out why my iPhone’s Face ID wasn’t working last night.
It’s my glasses.
I have two pairs of glasses. All through the work day today I was wearing my computer use glasses and Face ID worked every time. After work I switched to my regular use glasses and it failed. Huh. I took the glasses off and it worked. I put the glasses back on and it failed.
ADDENDUM: I cleaned the lenses on my general use glasses and Face ID works again. Whatdayaknow.