Power save mode, if you have it turned off, WILL COME ON by itself when battery level hits a certain threshold. Even if you have it off, it WILL come on by itself.
the setting is working normally. you can not fully deactivate power save mode from kicking in on a stock non rooted device.
I think you can go into the setting and check what the app manages when its on. I am going by memory on this...
clear as mud now?