All users obtain root privileges after logging in and are unable to exit, even if they are just regular users

In the Ubuntu 24.04LTS release, all users (including those not in the sudoers group) who log in and use commands such as whoami, id - un, and view UID will be displayed as root users with root privileges, even if the username displayed after logging in is a normal regular username. Therefore, ordinary users are able to write files as root in restricted directories (such as system folders or other user directories). This completely confuses the permissions of the entire system. Here's an example user:

getent passwd fsy
fsy:x:1001:1001:,,,:/home/fsy:/bin/bash

Does anyone know how to solve this problem ...