with "strange behaviour" I mean, that the selected points spring to the starting point of dragging and only then follow the mouse movement. I think it is due to the center rectangle point as reference, because if I start to drag in the middle of selected area (rectangle) then everything is ok (expected behaviour), but if I start to drag from other place of selected area, then the selected point spring there (strange behaviour) and after that follow normaly the mouse.

So which point shall be the reference point?