OK, I finally have some time, Phew.
I think that what Windows does when you do exactly what you did, is, when the displays are extended, it does not keep the Main monitor as the one you selected and/or the number changes.
I will tell you, it took me a LONG time to make it work consistently.
When you unclick the move to new monitor, be sure to click on apply.
When you then extend the monitor, be sure to examine which is the Main monitor at that time. I believe that is the main gotcha. Also, make sure that you right click on the desktop, click on View, and lock the Fences. If you then try to unlock the fences and move them to a different screen, all the stuff, at least on my client's computers, had to be restored. Immediately after you lock them, therefore, take a snapshot from which to restore if you get it wrong.
Be sure that you watch all the details carefully. I am sure you did, but I never could keep it all together until I became VERY careful on exactly what was happening.
I am willing, if you want me to, to monitor what you are doing by using Teamviewer. You can give me temporary access and then remove Teamviewer when I am off.
Really, Stardock has been totally unresponsive to this problem so far. They have a new Beta out but only available to the Dock 2016, which is way overkill for me or my clients. I do not know what they have addressed in the 3 Beta.
Let me know how everything works out.
Albert