I've taken an in-depth look at this. I have an iPhone 12 Pro Max. I investigated the issue and found out that the mail app drains the battery 100% when charging / at night. Later on I found out that actually the consumption rate is equal on the rest of the day, but due to the fact that I'm using other apps, the battery consumption is shared between them. So, main culprit: mail app, draining battery in the background, 100%. I just can't deal with this ATM, I can't remove the Mail app as I need it: I needed to recover my way up to iOS 16.5 unfortunately. The entire OS it's a bit too buggy at this time.