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.
GrapheneOS reboots once a day as a security measure.
As long as you dont turn that feature off it does. You can set it to restart if the device hasn’t been unlocked for between 10 mins and 3 days.