bug of OnCursorMouseDown

 I am using measurement studio 8.6(8.6.0.189) for C++, and when I use OnCursorMouseDown event,I found a weird phenomenon, the parameter CursorIndex return 0 when click cursor 1. But the help said: CursorIndex The one-based index of the graph cursor.

So I think this is a bug in version 8.6.0.189. Is it right?