Coming here from /e/OS
The issue is not totally fixed. Several of us are still experiencing the issue.
Ticket here: FP 6 v3.1.3 build random reboot when user travels between US and EU (#8895) · Issues · e / Backlog · GitLab
But this is interesting that the issue is happening on stock as well now.
I found out you can prevent the reboots by disabling location services, which has helped others too. See if that works. No GPS is no fun but it will probably stop the reboots.