KLR
Hello,
Right away, I'd backup your contacts, messages, photos, vanadium bookmarks, other messenger messages, etc
There's a new update going to "stable" in a few days. It could be worth it to change to alpha updates now or in a day to see if there are reported issues.
settings > system > system updates > change off of stable
Is it an option to have it on the charger before sleeping then take it off at 70% or so?
This could be a hardware failure. Before it happens several more times, I would store 2 copies of my backups, do the GOS update, if it still doesn't work, then flash GOS fresh again.
The phone turning off unexpectedly could be abrupt causing data loss and possibly stop it from booting up if the OS gets corrupted.
Wishing you luck.