Same phone, same beta, I had the same issue. I was able to reset the phone by briefly pushing first the up volume, then the down volume then holding the power button for 6 seconds. Upon release the Apple logo appeared and the phone booted. Didn’t have an issue again for about a week, then happened again today. Currently on iOS 16.0 (20A5358a), which I believe is beta 8. I hope Apple figures this one out before the launch next week.