Law enforcement believe the activity, which makes it harder to then unlock the phones, may be due to a potential update in iOS 18 which tells nearby iPhones to reboot if they have not been in contact with a cellular network for some time, according to a document obtained by 404 Media.
This is a better article with a link to the code. It has nothing to do with a software update (beyond being an unlisted feature of 18.1) and is based on an inactivity timer.
That’s great! I couldn’t find any mention of it on support.apple.com or on the iOS 18.1 details page.