I don't know if Fedora (or plasma) do things differently but the normal default is that you need sudo to reboot/shutdown/etc.
Then on basically all distros using systemd this behavior is modified via logind session and polkit. So the first thing to look at would be if a shutdown rule exists in /etc/polkit-1/rules.d/ that allow users in a certain group to shutdown/reboot in the first place.
Mine looks like this:
/etc/polkit-1/rules.d/50-shutdown.rules
polkit.addRule(function(action, subject) {
if ((action.id == "shutdown" ||
action.id == "reboot") &&
subject.isInGroup("wheel"))
{
return polkit.Result.YES;
}
});
So every user in the wheel group can do it without password prompt. Without this UI elements for shutdown/reboot will try assuming they have the right but simply fail.