This problem reared its ugly head on my phone today. I'm just ignoring the notification for now. Google doesn't allow turning off notifications for face unlock, but I've made it silent and minimized it, so it's as unobtrusive as possible. Face unlock still works for now, so we'll see how long it lasts. I'm hoping they include a fix for this in the February security patch, but not holding my breath. It's pretty ridiculous that only 3 months into the release of the phone they have already created a problem that at this point has no other resolution besides a factory reset. Never had this issue with the fingerprint sensor on my Pixel 1, 2, or 3.