It's a new feature of Android Pie and One UI, but even when updated, I've heard it doesn't work on some older models.
When you have screen lock on, but your phone is propped up sideways, it offers you the option to hit the button to switch to landscape and vice versa. It's irritating because it gets in the way of typing.
A shot in the dark, but it might help. Long press the blank space on your home screen, tap home screen settings, than make air keep on portrait mode only is on. Than in quick settings disable auto rotate. Again I haven't tested this out. If it worked for you, let me know.
I have figured out how to fix this.
1- Swipe down from the top of the screen to the quick menu.
2 - Click on the word portrait/auto-rotate below the icon and this opens up settings for auto rotate.
3 - Turn off rotate button on navigation bar.
Halle-fricken-lujah. No more pesky icon in the most annoying place!!!!
Thank you so so so much for this, this stupid dumb issue has caused me tremendous amount of unhappiness and grief. Honestly, it would haunt me every single day and Samsung couldn't solve it or even put it in the settings. I've considered switching phones, you just saved me <3.
I want to say a big thank you. This pop-up button has caused me so much annoyance, exceeded only by the frustration of not finding any way to turn it off. It's astonishing that they have a hidden "settings" section, but you have to click on the word rather than the icon. Thank you for finding and sharing this. I also created an account just to thank you.