I was having this problem as well, I turned off screen time. I then turned off my phone turned back on turned on screen time again and left my phone for 12 hours minus moving it and quickly checking the battery It went down 4%, it had been dropping 12% overnight.
I think it’s a bug with screen time and the longer it’s running the more resources it’s using that’s the only thing I can think of, after reading other users on Reddit having success with turning screen time off. I’m going to run it for a week and see if the battery starts getting worse again, then do a repeat of turning off screen time turning off the phone then re-enablng screen time when I turn the phone back on and see if it fixes it again. Its the only thing I can think of right now.
It could be of course that turning my phone off and on again stopped some runaway process that was not showing, my phone was perfect battery wise when I got it from new on release and now it’s back to how it was 4% in 12 hours not using the phone at all with Wi-Fi and cellular and data on is great much better than the 12% drop I was getting in 6-7 hours overnight before I tried this.