Just to support you: this behaviour has also been reported in the German section:
Markieren mit der Maus funktioniert nicht mehr so richtig
There, actually, the observation was that it is not a problem of the display number but of the way in which you move the mouse cursor: if you move it from bottom right to top left you see the behaviour. In other directions you don't. It is a "feature" of W10 as it does not happen with W7 or W8.