I am having the same issue. After I updated to IOS 14.3, my iPhone Xs Max drains battery super fast. I can wake up in the morning after having charged it over night within 3 hours, it has drained by 20%, after 5 hours it is down below 40%. Before the update, I was able to go all day without having to worry about the battery completely draining. Apple, please correct this. IOS updates should not drain batteries and then make a person think that they have to update their iPhone to the latest version because they think their current phone is failing. That is not fair practices. Thank you.