In Debian 12, the --user option in journalctl doesn't always work the same way it does with systemctl.

If you're using an older version of journalctl, you should use the --user-unit option instead.

If you don't get any results with the standard command:

$ journalctl --user --unit your.service

Please try the --user-unit option as follows:

$ journalctl --user-unit your.service