My wife got an 8 four or five weeks ago. I set it up for her at the time, restored it from a backup of her previous 6S, and all was well until a few days ago.
She always charges it at night and starts out the day with a full charge, and honestly doesn’t use it that much at work. However, whatever happened, she was was suddenly coming home with only about 25-30% left in the battery, rather than the usual 70-80%. I updated it to the latest version of iOS, but no better. I’ve turned off as much as a can think of, tried resetting the network settings, but no better. Today we saw the battery drop from 50% to 4% in approximately 90 minutes. The phone wasn’t even being used in any way during that time.
This evening I have wiped the phone, and set it up as new to see what happens. Seems odd it was behaving perfectly up until a few days ago, then seems to have gone a bit haywire. There was nothing changed about the phone (new apps etc.) to explain the dramatic change.