Why can the Xiaomi 15 Ultra be unlocked by someone else’s fingerprint?

Thank you for your support for the Xiaomi product. We apologize for the inconvenience.
Because fingerprint and face unlock or Bluetooth device unlock are turned on at the same time on the phone. When your friend or colleague unlocks the phone with his fingerprint, you are photographed by the camera, and then unlocked by face or Bluetooth device. It is recommended to keep the phone at a certain distance from your face when unlocking.
If it is confirmed that it can still be unlocked by someone else’s fingerprint without turning on face unlock and Bluetooth device unlock, there is a low probability that it can be caused by the fingerprint mark composed of dust or stains left on the screen when someone else’s fingerprint unlocks.
For your reference, here are the steps to troubleshoot:
1. Delete all recorded fingerprints, face unlock information and Bluetooth device unlock information;
2. Wipe the screen with a tissue or cloth, and then re-record your fingerprint;
3. After recording, wipe the screen with a tissue or cloth. Do not record the face unlock information first, and then try to let a friend or colleague who could unlock your phone unlock it.
If you find that others cannot unlock your phone after trying the above steps, it means that your finger will leave marks on smooth surfaces such as glass. When the fingerprint mark is thick and obvious, the fingerprint sensor will recognize some information in some special scenarios, resulting in a small probability of being unlocked by someone else’s finger under special circumstances. It is recommended that you wash your hands frequently and wipe the fingerprint recognition area occasionally.