So, something has changed in the way lightdm-gtk-greeter is packaged and works. The work around is to do the following:
sudo cp /etc/lightdm/lightdm-gtk-greeter-ubuntu-mate.conf /etc/lightdm/lightdm-gtk-greeter-ubuntu-mate.conf.d/50_ubuntu-mate.conf
So, something has changed in the way lightdm-gtk-greeter is packaged and works. The work around is to do the following:
sudo cp /etc/lightdm/ lightdm- gtk-greeter- ubuntu- mate.conf /etc/lightdm/ lightdm- gtk-greeter- ubuntu- mate.conf. d/50_ubuntu- mate.conf