lightdm install prompts about default dm even though debconf value set

I've got gdm3-3.18.3-0ubuntu2 installed on a Xenial 16.04 system with apt. Now I need to install lightdm-1.18.2-0ubuntu2 as well and also need to avoid being asked any questions. But the system asks me, what default DM (display manager) to use.

Copying from Bug #1020770, I set some values with debconf.

$ echo "debconf debconf/priority select critical
lightdm shared/default-x-display-manager select lightdm
gdm3 shared/default-x-display-manager select lightdm" | sudo debconf-set-selections -v
info: Trying to set 'debconf/priority' [select] to 'critical'
info: Loading answer for 'debconf/priority'
info: Trying to set 'shared/default-x-display-manager' [select] to 'lightdm'
info: Loading answer for 'shared/default-x-display-manager'
info: Trying to set 'shared/default-x-display-manager' [select] to 'lightdm'
info: Loading answer for 'shared/default-x-display-manager'

When I now install lightdm (or reconfigure it), it is ignoring that I set the value to "lightdm". I confirmed that by having a look at /etc/X11/default-display-manager:

$ cat /etc/X11/default-display-manager
/usr/sbin/gdm3

When I manually answer the DM question and give "lightdm" as the answer, the file contains "/usr/sbin/lightdm".

dpkg-reconfigure in non-interactive mode:

$ sudo env DEBCONF_DEBUG=developer dpkg-reconfigure -fnoninteractive lightdm
debconf (developer): starting /var/lib/dpkg/info/lightdm.prerm upgrade 1.18.2-0ubuntu2
debconf (developer): starting /var/lib/dpkg/info/lightdm.config reconfigure 1.18.2-0ubuntu2
debconf (developer): <-- METAGET shared/default-x-display-manager owners
debconf (developer): --> 0 gdm, gdm3, lightdm, slim
debconf (developer): <-- METAGET shared/default-x-display-manager choices
debconf (developer): --> 0 gdm, gdm3, lightdm, slim
debconf (developer): <-- SET shared/default-x-display-manager gdm3
debconf (developer): --> 0 value set
debconf (developer): <-- INPUT high shared/default-x-display-manager
debconf (developer): --> 30 question skipped
debconf (developer): <-- GO
debconf (developer): --> 0 ok
debconf (developer): <-- GET shared/default-x-display-manager
debconf (developer): --> 0 gdm3
debconf (developer): starting /var/lib/dpkg/info/lightdm.postinst configure 1.18.2-0ubuntu2
debconf (developer): <-- STOP

And in interactive mode:

$ sudo env DEBCONF_DEBUG=developer dpkg-reconfigure lightdm
debconf (developer): starting /var/lib/dpkg/info/lightdm.prerm upgrade 1.18.2-0ubuntu2
debconf (developer): starting /var/lib/dpkg/info/lightdm.config reconfigure 1.18.2-0ubuntu2
debconf (developer): <-- METAGET shared/default-x-display-manager owners
debconf (developer): --> 0 gdm, gdm3, lightdm, slim
debconf (developer): <-- METAGET shared/default-x-display-manager choices
debconf (developer): --> 0 gdm, gdm3, lightdm, slim
debconf (developer): <-- SET shared/default-x-display-manager gdm3
debconf (developer): --> 0 value set
debconf (developer): <-- INPUT high shared/default-x-display-manager
debconf (developer): --> 0 question will be asked
debconf (developer): <-- GO
debconf (developer): --> 0 ok
debconf (developer): <-- GET shared/default-x-display-manager
debconf (developer): --> 0 lightdm
debconf (developer): starting /var/lib/dpkg/info/lightdm.postinst configure 1.18.2-0ubuntu2
debconf (developer): <-- GET shared/default-x-display-manager
debconf (developer): --> 0 lightdm
debconf (developer): <-- GET lightdm/daemon_name
debconf (developer): --> 0 /usr/sbin/lightdm
debconf (developer): <-- STOP

To me, this is a bug in lightdm (or gdm3, which has the same code and problem), because it doesn't seem to honor the values set with debconf and thus makes it impossible to install without having to manually answer questions.