Trying to switch off the FP2 it just reboots even the Switch off button has been pressed.
As I just received a new FP2 a week ago for a family member we so the same behavior with the new phone after upgrading it to the above mentioned OS 18.09.2
This new phone had no sim-card and no sdcard installed and shows the reboot instead of switch off behavior. Only by removing the battery at the moment after the vibration for the switch off the phone stays off when the battery is inserted again.
Though both FP2 (one received last week, one bought about 2 years ago) show the same behavior after the upgrade. The updater says the OS is up-to-date.