Your comment about glasses helped solved the mystery. I recently switched to progressive lenses and as a result have to look at my phone through the lower half of my glasses. When I do so the phone doesn’t unlock. Once I move it to the mid-range or distance top half it unlocks. I just tried this multiple times. I’ll just have to get use to unlocking it using the distance vision half and then shifting it down to the reading lens portion to use the phone. ? (the other thing I noticed is that I need to look at the middle of the iPhone screen, not directly at the faceId camera)I noticed mine became a little fussy after I applied a full surface screen protector. But, it’s only an occasional fail and seems to have gotten better with time.
My biggest problem is forgetting to look at the sensor when I take my glasses off. Otherwise, tilting toward my face with glasses on unlocks very consistently. When I put on a mask, unlock with Apple Watch has been very reliable.