I had this, last night. My iPhone 13 Pro appeared to go off around 0100 and was still in „Sleep“ Focus in the morning (my Apple Watch, fortunately, woke me up).
The iPhone stubbornly thought it was still in „Sleep“ mode (including blanking the wallpaper off the Lock Screen) until I rebooted it at about 0900.
iPhone appear to turn off for a while overnight? You’re not alone (9to5mac.com)