Apple added a feature called "inactivity reboot" in iOS 18.1. This is implemented in keybagd and the AppleSEPKeyStore kernel extension. It seems to have nothing to do with phone/wireless network state. Keystore is used when unlocking the device. So if you don't unlock your iPhone for a while... it will reboot!
In the news: "Police Freak Out at iPhones Mysteriously Rebooting Themselves, Locking Cops Out"
https://www.404media.co/police-freak-out-at-iphones-mysteriously-rebooting-themselves-locking-cops-out/
iOS version diffs to see yourself:
https://github.com/search?q=repo%3Ablacktop%2Fipsw-diffs%20inactivity_reboot&type=code
@jiska at least police have effectively confirmed for us that they keep seized phones powered up in a Faraday cage
@mikelabonte Not really new. Faraday bags are part of the Cellebrite toolkit https://cellebrite.com/en/from-murdaugh-to-josephson-how-south-carolina-law-enforcement-division-is-taking-on-big-cases-with-digital-intelligence/ @jiska