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iPhones stored for forensic analysis unexpectedly reboot, causing problems for officials
(appleinsider.com)
This is a most excellent place for technology news and articles.
There is a shortcut action to shut down the phone which you could trigger with an automation, I suppose.
I can put the phone in lockdown by press-holding the power button and select the "lockdown" option. Not bad, but an actual reboot daily at night wouldn't be so bad.
Samsung has this option.
GrapheneOS has the option for a scheduled reboot if the phone hasn't been unlocked for a configurable amount of time.