Let us help you. Please tell us about...
Affected device: mobile phone
Operating system: Android 8.1.0
Part of Prey not working: Locking off button
Could you please describe the problem?
When the option is checked on phone a PIN is requested when you try to turn off the device, If you try to turn it off again while the PIN is asked, you can turn the device off without any authentification.
Even if you lock the device after the PIN is requested, you are able to turn it off from the locked screen.
It seems like a serious problem with these feature as a powered on phone cant be tracked
And finally, please attach a screenshot if available (you can drag and drop images)