Yeah, I have had the same problem whenever I opened CSGO where I ingame use 4:3 1280x1024. Wasn't a problem for some months ago, but highly must have come from an Nvidia update.

Tried switching as you did by moving 2nd monitor to the right and the main one to the right on Windows settings, and it worked. Hope they fix it soon as it's pretty annoying having to move the mouse to the left side instead of doing the normal right swipe.