The phone has turned on the face unlock function. However, when the phone has a message notification or reminder on the screen, the face unlock function cannot be used to unlock the phone.
It's a security design of the Android system. To avoid automatical initiating face unlock that results in unintentional unlocking of the phone, which may affect your privacy and security, therefore, when the phone has a message notification or reminder to brighten the screen, you cannot use the face unlock function to unlock the phone. This is not an abnormality of the phone, please feel free to use it.