Currently GraphicsDevice#getDisplayMode() does not receive mode from native platform in case of Xinerama and returns the default mode form getBounds():

        Rectangle r = gc.getBounds();
        return new DisplayMode(r.width, r.height,
                               DisplayMode.BIT_DEPTH_MULTI,
                               DisplayMode.REFRESH_RATE_UNKNOWN);

The corresponding native code need to be fixed to return the actual mode,