I've experienced this. Reloaded and setup as new since I didn't do that when I first got my 6s. Problem was still happening. Last night I let it die completely before charging it. I'll see if it fixes it.
Letting the phone discharge until it shuts itself down then plugging it until it fully recharges lets the meter readjust to the battery's loss of capacity with age. Doing that two or three times a year has been said to have beneficial effects on battery life. I've been doing that with my phones since my first one and have never had battery issues even on my iPhone 3G which was about four years old when I finally retired it to the great Apple orchard in the sky.