Comment 19 for bug 1801609

The cause of this issue has to be in Xorg 1.20 then (18.04 comes with 1.19).

I assume that xserver-xorg-video-intel is still installed by default, so please check if removing this driver package has any effect (the generic modesetting driver will be used instead).

For reference the greeter code:
https://git.launchpad.net/lightdm-gtk-greeter/tree/src/lightdm-gtk-greeter.c#n2830