Uh, shutdown -p is the poweroff one, but that's still interesting. After a normal shutdown and then boot, does the shutdown button in the webUI still reboot the system? (Grasping at straws here. My best guess would be that this is something hardware specific.) For your own purposes, check if 'shutdown -p now' does shut down the system properly, and if it does, I guess use that from the web shell from now on instead of the GUI button.