Auto reboot can be configured under Settings> Security & privacy> Exploit protection> Auto reboot. This menu item can only be accessed from owner profile; however, an auto reboot also takes place regardless of which profile you are logged into.
The following things come to mind off the top of my head:
1.) Has it been 18 hours since one of your profiles was last unlocked?
See:
GrapheneOS provides an auto-reboot feature which reboots locked devices after a set period of time to put data at rest. A countdown timer is started each time the device is locked, and the device will reboot if a successful unlock doesn't occur before the timer reaches zero. Unlocking any profile cancels the timer, not just the Owner profile.
The timer is set to 18 hours by default, but can be set to values between 10 minutes and 72 hours, or turned off.
This feature doesn't apply when the device is in "Before First Unlock" state, meaning that it will not lead to the device continuously rebooting, as data is already at rest.
source: https://grapheneos.org/features#auto-reboot
2.) Maybe I'm misunderstanding, but your wording
...asking to reboot
sounds more like the notification that appears when a new GrapheneOS release has been installed and needs your device to reboot.
If Settings> System> System update> Automatic reboot is toggled off, a notification asking you to reboot would show up which can be confirmed or canceled.
However, this setting only refers to system updates, not to Auto reboot, a security feature which automatically brings your device into BFU making it harder to exploit. More infos about AFU/BFU can be found here:
https://discuss.grapheneos.org/d/12848-claims-made-by-forensics-companies-their-capabilities-and-how-grapheneos-fares
https://cellebrite.com/en/episode-23-i-beg-to-dfir-data-extractions-explained-ffs-afu-bfu-advanced-logical-digital-forensics-webinar/