Tried this again:
Tap to wake off
Wi-Fi off
4G off
Bluetooth off
Screen time off
Hey Siri off
Low power mode OFF
Background app refresh off
iPhone XS Max 50-49%
Seems like there’s no issue with the phone entering the idle state. Wi-Fi/4G on seems to drain more battery than it used to in previous iOS versions.
My iPad Air 2 with 100% battery health also sometimes drains around 4% overnight while on Wi-Fi. Using iMazing, I can check current mAh usage remotely via Wi-Fi. I should expect to see between -5 and -10mAh, however sometimes it will get stuck at -80mAh, which correlates with the iPad draining 4% overnight. A rogue app, maybe?