Comment 0 for bug 1807197

gdm3 under Ubuntu 18.04 amd64 doesn't save/restore the last session you logged into, instead it always reverts to GNOME (that's the case if you have GNOME installed at least). If you want to user any other desktop it has to be manually chosen from the session menu every time you login.

It should be noted that I use pbis-open (PowerBroker Identity Service) for AD authentication, because its a work machine, hence the first two lines of my /etc/nsswitch.conf look like:

passwd: compat systemd lsass
group: compat systemd lsass

and my login/username looks like:

domain\user

ie With a backslash in the middle.

If this is not a bug and gdm3 isn't supposed to default to using your last used session/desktop, how do I force gdm3 to default to MATE instead of GNOME?

Thanks