Hi there,
I reported the same issue two days ago:
https://nothing.community/d/48665
Support told me that this type of behavior—a sudden deep sleep followed by an unprompted return to normal function with minimal battery change—can sometimes occur after a major OS update while the system stabilizes and re-indexes.
They suggested to monitor it, and if that happens again, to send them the system logs.
My phone switched off for approx. 30 mins. no matter which button I pressed, it was dead.
After 30 mins. it woke up again and working fine.
I hope that was an isolated event
Alex