I’ve also seen issues where full-GUI systems fail to reboot if there are logged-on, but disconnected, RDP sessions. I believe this may be the same cause as Core editions - the BES Client is trying to start the pop-up GUI to warn the logged-on user about the reboot, cannot display the GUI, and does not continue the reboot.
Once in this state, if a user logs on or reconnects later, the system immediately restarts.
I saw a client setting here in the forum a while back that can be applied to force the reboot without opening the client UI. I’ll see if I can find it.
Edit: found it …