do this and the problem is solved
On the File Manager:
1. Go to Settings -> Screen -> Rotation and check "keep current rotation across views"
2. In order to prevent "magic stuff" make sure you exit the program in Hamburguer -> Exit just after that.
3. Open the program again, go to Settings -> Screen -> Rotation and select your preferred rotation.
4. In order to prevent "magic stuff" make sure you exit the program in Hamburguer -> Exit just after that.
As you can see it could be a single step "check keep rotation across views and select rotation". But then, again, would make the problem trivial, so please follow all four steps in the given order
Extra hints: the 90º steps you see have a nice arrow that tell you how you'll need to move your device to adapt to the new screen rota.
Have fun!