"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
…
Apple added “inactivity reboot” code in iOS 18.1 that triggers iPhones to restart after they’ve been locked for four days"
You don’t have to reboot it. You just have to click the main side button several times (10?). Which you can do by feel.
5 times locks it.