This is exactly how Face ID functions according to the white paper released by Apple.
Face ID will periodically use successful scans of your face to add to the matrix. On failures to recognize, but then after a successful passcode entry and within a certain threshold, it will mess those copies in to learn drastic changes faster.
https://images.apple.com/business/docs/FaceID_Security_Guide.pdf
"The following Face ID data is saved, encrypted only for use by the Secure Enclave, during normal operation:
• The mathematical representations of your face calculated during enrollment.
• The mathematical representations of your face calculated during some unlock attempts if Face ID deems them useful to augment future matching."
"To improve unlock performance and keep pace with the natural changes of your face and look, Face ID augments its stored mathematical representation over time. Upon successful unlock, Face ID may use the newly calculated mathematical representation—if its quality is sufficient—for a finite number of additional unlocks before that data is discarded. Conversely, if Face ID fails to recognize you, but the match quality is higher than a certain threshold and you immediately follow the failure by entering your passcode, Face ID takes another capture and augments its enrolled Face ID data with the newly calculated mathematical representation."
Very helpful. Thank you both.