This problem occurs when Wayland is enabled on the remote Linux computer. When the Linux system has its monitor configured in portrait mode, the client system (e.g. Windows) views that monitor in landscape mode only. As a consequence, the NoMachine session screen is rotated of 90°.

A possible workaround is to disable Wayland on the remote Linux.

This issue has been solved in version 8.17.1 for KDE Plasma 6 but not for previous versions of KDE Plasma using Wayland, due to the fact the system wasn't yet providing the needed info.