I've had this issue for several months. I have an iPhone 11 Pro Max with iOS 13.1.3. In the last 10 days, the Photos app is being reported to have used 84% of all battery usage (showing as 75 hours and 33 minutes) IN THE BACKGROUND, with only 71 minutes actually on screen...***?!
You're right, the Photos app doesn't show in the Background App Refresh list so that advice is wrong. Also, the Photos app still shows as using battery in the background when I don't even have it running in the background...in fact, it still shows when I have no apps at all running in the background.
Other than reset my device or upgrade to the latest iOS (neither of which I want to do), I've tried every suggestion I could find on every forum I could find. It seems that many people have this issue but nobody has actually found a solution.