It is not necessarily an issue on the App side, it could be the fault of the device manufacturer's contribution to Android. As Anke showed in an earlier post, some devices have their own setting for screen orientation, independent of the Apps, so everyone should disable that (if possible). Of course, the fact that recently the Companion Screen sets itself to Landscape if the phone isn't held in portrait (or is simply not being held) might be a clue that the bug is on the App side, since this behaviour is relatively new (nb190?)
I had not noticed the issue until this Topic was raised because usually an App is portrait mode and I hold the phone in portrait, so I don't think it is really sooo bad.