Same problem! I have an iPhone 7S running 13.6. Problem started about a month ago. Might have been concurrent with recent iOS update, but I was not tracking the timing. Phone got super hot and battery drained in about an hour. Determined Mail was running non-stop. All background refresh is off, all fetch is off, I finally deleted the Mail app entirely. That solved the problem for about a week. Battery lasted 2 days without charging. Now iPhone is red hot and drained 25% in a half hour this morning. Pandora seems to be the culprit (background activity) - but the app is closed and confirmed Background Refresh is off. I've used Pandora almost every day since this battery drain issue arose, and it has not been a factor until today. I'll try a complete Reset and Restore next. Not optimistic - seems to be an iOS problem.