Unfortunately, I have exactly the same problem. The second screen is also a touch notebook with a built-in orientation sensor. I am using the Windows store viewer on Windows10.
I played around with the settings for 2 days and found a workaround that worked for me.
The problem is indeed that the viewer thinks it is in portrait mode. So, the workaround was to set the client display of the second notebook to portrait mode, then start the Spacedesk Viewer connection, and then set the display settings back to landscape mode.
I hope this helps some of you here in search of a solution.
The problem could simply be solved with a “Set orientation” function in the viewer app.
