I posted earlier about my iPhone 7+ having this issue and after playing with my settings I think I’ve found it. For me it is having my cellular data on while on WiFi seems be causing the problem. I was looking at every possible setting and found a few apps I only ever use at home had jumped to more data usage than they should be for just notification pings. So I turned it off and left my phone for an hour and only a 1% drain. I also tested having cellular data on when I’m out of a WiFi area and the battery drain is normal. So my drain seems to be those 2 fighting to see which should be used. I hope this helps a few people.

My 7+ is also having the same issue, but it is even weirder. If I restart my phone it jumps back up to 98% when it was down at 50% after a half hour use. I think it’s more of bug with the iOS versus the device.