And it's going to continue to drive you nuts until you install Displayfusion. Windows has always had both random and persistent issues with various aspects of their multi monitor support. I'm very surprised you haven't had issues prior to this if you've had this configuration a while.
Every time you update the display driver, or connect/disconnect displays, and ten other triggers, all cause weird crap like icons moving, wrong display being configured as left/right for mouse movement, applications opening on the wrong display, and so on.
Since installing the free version of Displayfusion I've never had any issues with anything related to having multiple monitors ever again.
The free version never expires, and while it lacks some features of the paid version they are not anything that I've ever missed but if you need very advanced options for multi monitor configuration, it's there for those who require it. For the rest of us, the free version does basically everything that Windows, Nvidia, AMD SHOULD do natively, but doesn't.
DisplayFusion will make your multi-monitor life much easier! With powerful features like Multi-Monitor Taskbars, TitleBar Buttons and fully customizable HotKeys, DisplayFusion will make managing your multiple monitors easy.
www.displayfusion.com