Both sessions are Ubuntu on Xorg in 18.04, even though one is ubuntu.desktop, and the other ubuntu-xorg.desktop.
Are you experiencing a specific bug? Note that there is also /usr/share/wayland-sessions/
/usr/share/wayland-sessions/
only shows,
Does the GDM login look in /usr/share/xsessions or in /usr/share/wayland-sessions? or looks in both? If Lightdm is exchanged for GDM, the default is only Ubuntu on Xorg.
Ok, please file a bug against LightDM since I believe it is working fine with GDM.
Interestingly, this might be a regression for LightDM introduced by an Ubuntu 17.10 gnome-session update. Try to see if you can reproduce from fresh Ubuntu 17.10 without updates and again after applying the gnome-session updates.
It is working fine with GDM, but GDM is somewhat slow in starting, after the login unlock is clicked. It is like, GDM is sort of undecided, whether to start or not.