This is an interesting problem and I am studying how to solve this. Essentially screen mirror thinks the camera app is in portrait mode…because it is. On android and iOS devices the camera is a portrait application that does not change orientation when you rotate the phone, it just creatively rotates the text labels.
This is just like the app launcher which doesn’t care how you hold the phone, it shows the icons in a portrait mode.
There isn’t a clean way to detect that the Camera app is running and rotate the screen. I hope to find a fix sometime soon.