In further testing, I've moved the lightdm upstart definition out of the way and updated /etc/X11/default-window-manager to point to /usr/sbin/gdm. This also rectifies the problem.
Simply having both upstart definitions present seems to create the problem. Going to test with additional DMs to see if the problem persists as long as there are two DM upstart definitions present.
In further testing, I've moved the lightdm upstart definition out of the way and updated /etc/X11/ default- window- manager to point to /usr/sbin/gdm. This also rectifies the problem.
Simply having both upstart definitions present seems to create the problem. Going to test with additional DMs to see if the problem persists as long as there are two DM upstart definitions present.