I know it sounds crazy, but yes, the admin group is gone on the machine after a Big Sur upgrade. Also, the user cannot get into recover as his pwd is not accepted. Passwords are not prompted for. Keychains are not available. The Wifis cannot be accessed. I cannot reproduce this. Has ANYONE seen this? The user found an article saying that a SMC reset is the end-all fix which.... didn't work.
Any A/V software installed on the device? I ask that because in years past, I've seen the local admin group get removed, but it turned out to an issue with McAfee, not actually an upgrade of the OS.