This can happen for various reasons.

Or maybe you wear glasses sometimes and use contacts other times.

Scroll down and selectFace ID & Passcode.

Screenshots illustrating how to get into Face ID & Passcodes on iPhone.

key in your passcode to be taken to yourFace ID & Passcodesettings.

you might exit that, which will automatically activate your new Alternate Appearance.

Be sure you trust the person you’re adding as an Alternate Appearance.

Screenshots of setting up an Alternate Appearance on iPhone.

Screenshots of finalizing the face scan for an Alternate Appearance on iPhone.