Hard reboot your iPhone and go back into Face ID and Passcode, the hard reboot will fix your Face ID Issue - then turn off Stolen Device Protection - SDP really complicates issues.
I was just on the phone with Apple for 45 minutes, they told me to wipe the device; I did not want too wipe the device, restoring from a backup would bring the issue back in and setting up new takes an hour. I support a large number of iPhone users, I ALWAYS do a hard reboot before calling Apple - I forgot to do it with a user this time. Very happy I thought of it, the hard reboot fixed my issue; the Apple Support documentation did not include this information and most people would not think about doing it.