Most of my usage was music playing, some web browsing on mostly on LTE and text messaging a lot throughout the whole time.
My brightness is to auto, LTE and WIFI are set to on. Bluetooth is off.
I did use "Do not disturb" at work from 9 AM to 5 PM but I was still constantly unlocking it to change songs and reply to texts.
but where is your brightness slider set too? and did you restore from backup or as new??