Can I assume that you're using the GS5? If that's the case, then it's a function of your Power Saving mode. More specifically Ultra Power Saving. Tweak or customize your Power Saving options to correct it.
I just did it too, the bottom button on the side is a quick use button that can be assigned an app. Mine was set default to engage reader mode -turning the screen b/w and locking it in portrait view. Hit it again, hold it to reassign to something less annoying
I think that every answer that can be given, has been given. I don't think that we have anything else to suggest, apart from seeking advice directly from the manufacturer via their customer support site.