Shuts down unexpectedly at login screen, but not after login.

This iMac shuts down and turns completely off at the login screen after about an hour. However, if I log in to the admin account, it stays on -- I left it overnight with no apps running and it never even went to sleep. It's running Ventura 13.4.1 (c). I can't find a setting that causes a shut down only at the login screen. There are no "after-login" settings that would cause it to shut down while logged in. It passed built in Apple diagnostics and the file system integrity test. It has a 250GB SSD and 138GB free space; there is no malware ... that Malwarebytes could find at least. It does only have 8GB RAM, but I've never seen that cause a shutdown at the login window only. What could be causing it could shut down automatically like that?