This reproduces only if RadRibbonWindow is used with the default WPF Window's theme, instead of the Telerik's theme. Also, this reproduces only in the themes after Expression_Dark (starting from Windows8). You can find a list of the Telerik themes ordered chronologically in the help documentation.

If the RadRibbonWindow is maximized on the second (right) monitor and you click the application menu button to open the ApplicationMenu, the menu displays on the first (left) monitor.

To work this around enable the Telerik's RadRibbonWindow theming by setting the IsWindowsThemeEnabled static property to False.

static MainWindow()
{
    IsWindowsThemeEnabled = false;
}