"There is an apparently new iOS 18 security feature that reboots iPhones that haven’t been unlocked in a few days, frustrating police by making it harder to break into suspects’ iPhones
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Apple added “inactivity reboot” code in iOS 18.1 that triggers iPhones to restart after they’ve been locked for four days"
This disables biometrics, but doesn’t delete the decryption key in memory. This is sufficient to prevent being compelled to unlock with biometrics, but with the decryption key in memory, it is still possible to access data via an exploit. A full reboot deletes the decryption key until the user enters their pin.