one fingerprint allows two different people to login

Here’s a weird bug: My wife got her MacBook Air M1. She set it up and enrolled her fingerprint during the initial startup. We locked the screen and she used her finger to unlock. Yay! Then we locked the screen and i placed my finger on Touch ID button---and it unlocked ?!? Build is 20C69, there is only one registered fingerprint (wife's). 100% reproducible, and persists after reboot. Is this a security hole or statistical fluke?