I wouldn’t be expending too much energy diagnosing hibernation faults on OSes that have actually been finished, personally. It’s always been problematic. On a beta product, who knows.
Just to be clear though, you say it also does it from cold? So, what, you switch it on, it boots, starts the login sequence, spontaneously reboots and then works fine on that second boot? Or is it just when suspend to disk is involved?
Basic checks like a chkdsk would be my first port of call. Check for driver updates, that sort of thing. Maybe disable / re-enable hibernate, I’d expect that to recreate the hibernation file (but I don’t know for sure).