lightdm-settings doesn't start

Here is what i tried:

$ apt install lightdm-settings
$ grep Exec /usr/share/applications/lightdm-settings.desktop
Exec=pkexec lightdm-settings

$ pkexec lightdm-settings

[... GUI opens asks me to enter my password ...]

Error executing command as another user: Not authorized

This incident has been reported.

a policy is delivered with the package:

$ dpkg -L lightdm-settings|grep .policy
/usr/share/polkit-1/actions/org.x.lightdm-settings.policy

polkitd is running:
$ ps auxw|grep polkitd
root 37836 0.0 0.0 242172 11088 ? Ssl 20:33 0:01 /usr/lib/policykit-1/polkitd --no-debug

is this a bug?

(Lubuntu 20.04)