GrapheneOS has a default enabled auto-reboot feature which starts a timer after the device is locked and reboots it if the timer elapses before the next successful unlock. The default is 18 hours to avoid causing any reboots for most users, but users can set it as low as 10 minutes or as high as 72 hours.
There's a table in Cellebrite documentation specifically for Pixels comparable to the iPhone table which includes GrapheneOS. Look here:
https://grapheneos.social/@GrapheneOS/112462758257739953