With the 'keep ratio' box checked, the picture aspect ratio is not maintained while adjusting the picture size by dragging the picture border (edge or corner with little green squares). I just discovered that the aspect ratio is maintained while adjusting size inside the picture dialog box. 'keep ratio' should also apply while dragging image size.
this checkbox indeed is only for automatic calculation of the 2nd value for size in the dialog.
If you want to keep ratio for mouse operations press <shift> when dragging control points.
*** This bug has been marked as a duplicate of bug 84565 ***