TDE isn't shutting down when LightDM is the default (actually only) login manager. Multiple machines (x86 and x86-64) with Debian Stretch fully up to date using the Preliminary Stable Builds repository and LightDM as the login manager. Go to shutdown the machines and TDE merely send the user back to LightDm (as though they only selected log out) where they have to shut the machines down in LightDM.