Apple indeed 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. github.com/search?q=repo%…
There's a new article with more details on when the new inactivity reboot is triggered on iOS 18. This is a cheap & great mitigation! While most people won't have their phone forensically analyzed, many more will have their devices stolen. It protects user data in both cases.
Hope I can give you more details on inactivity reboot in ~4 days 🤞
Strings are a good starting point, but it’d be nice to see some logic. Has reboot logic been found?