I used to have my screen orientation locked to portrait because that's what I use most of the time. If I needed to use landscape, I could easily change the setting by pulling down the icons from the top and clicking the screen orientation auto rotate icon. For some reason, I can't lock it in to portrait mode now. Each time I turn off auto rotate, within a few minutes, it resets to auto rotate on. I know we had an update shortly before this started. Perhaps that caused it. How can I make the phone screen stay in portrait mode until I decide I want to manually set it to auto rotate?
This seems like a software issue you will have to back up your data and do a factory reset. If your problem still continues please contact me. Please make sure you know your google account information on your device before doing this.
Here is a link provided by samsung on how to do a Factory Reset: