This is just a theory!
So unlike many of the short YouTube videos claiming that FaceID is insecure and flawed, I will show a front to back example of a TouchID issue that could be somewhat related to FaceID learning 2 faces under one profile.
First off, this is NOT an official MacRumors video, I am just a member that volunteers to moderate, and this channel is only used for my posts on MacRumors.
It is my belief (granted with no proof) that FaceID is being trained similarly to my example, due to it's apparent attempt to learn a rejected face after a password is entered.
Apple said:
https://images.apple.com/business/docs/FaceID_Security_Guide.pdf
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.
Essentially, it is my feeling that faces of siblings and relatives are being programmed in by this learning feature. While different technology on the front side, I have demonstrated that the secure Enclave can be trained to accept 2 different biometric inputs (two different fingers) under one profile.
The TouchID secure enclave could learn two different fingers as one, and if FaceID shares any of that machine learning algorithm , I don't think it is a stretch to think that many of these FaceID "family hacks" are the result of accidental (and sometimes intentional) programming.
Be curious to see if we ever get any acknowledgment from Apple on this though, as all of this silence is doing little to keep people confident in this tech.
Take what I posted with a grain of salt, it is only my opinion on FaceID, but I have done this hack with TouchID for years now on my 5S.