PrepareForShutdown() signal from logind is not handled
When a user logged in to the GUI session and she/he request shutdown of the system a PrepareForShutdown() signal is emitted by logind:
https://www.freedesktop.org/wiki/Software/systemd/inhibit/
Processes holding delay inhibitor locks can start their pre-shutdown operationsand the Gnome 3 session does handle the signal and logs the user off to GDM, but GDM does not tell that shutdown is pending and the user may think that she/he can log in again or should to something to really start the shutdown.
With systemd's default configuration the confusion only lasted up to 5 seconds (default max delay for inhibitor locks), but when the max delay is increased either by a package or in local configuration the confusion can last longer.
Please consider either stopping GDM to switch over the shutdown screen or at least showing a notification about the ongoing shutdown.
The issue was originally reported at https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1803581 .