Comment # 5 on bug 1197610 from Johannes Segitz

The changes in the .service file cause this, specifically

ReadWritePaths=\
  -/etc/gdm/custom.conf \
  /etc/ \
  -/proc/self/loginuid \
  -/var/log/lastlog \
  -/var/log/tallylog \
  -/var/mail/
ReadOnlyPaths=\
  /usr/share/accountsservice/interfaces/ \
  /usr/share/dbus-1/interfaces/ \
  /var/log/wtmp \
  /run/systemd/seats/

this causes the mount operations to happen