BUILD ID: 188.8.131.52
HARDWARE: Xperia 10 DualSIM
UI LANGUAGE: German
This is an old problem of SFOS, I had it also for many years… but since the last update to SFOS 184.108.40.206 it is much better than before but not yet fixed completely.
Was your phone plugged to the charger as you tried to switch it off?
Just now I tested to switch off my phone to see what happens. Result is:
If the charger is plugged in, then the phone shuts down and then reboots.
If the charger is disconnected, the phone shuts down and stays off (does not reboot)
As said, this is an old problem and it means not, that there is a major error. You can confidently ignore this, and if you really want to switch off the phone, then unplug the charger.