It's your screen brightness. You've isolated the issue to this point: it isn't faulty hardware, you got your phone replaced. It isn't your backup, you set your phone up as new. It isn't that too many battery-hog settings are on, you've turned them all off. Stop using your phone at 100% brightness. The 6s+ has a larger screen to illuminate, 100% brightness will wreck your battery, and what about your eyes? Doesn't that bother your eyes? If my iPhone 6 is higher than 75% in an indoor environment it's too much for me, and if it's at night, forget it. It freakin hurts. Plus my iPhone 6 warms up really bad if it's higher than 50% or so, and the battery tanks with like 3 and a half, 4 hours of use before its dead. That's with a smaller display to power as well. Try running at 30% brightness and tell us how bad your battery is.I assume the 24 hour thing is based off of when I exchanged/had the device reset as new. However, I have absolutely no idea why the battery stats rolled over...
edit: fifteen percent left, at 4 hours of usage.
I swear I loose a percentage or two every minute...even just browsing safari.