layoutStretch is applied in Qt Designer but not in the preview window

[AlbyTree]

Hi. I have this simple window in Qt Designer with a vertical layout and some horizontal layouts, each of which contains some widgets.
I set layoutStretch 25,25,50,0 of the vertical layout and it is correctly shown in the designer window

When I preview the window, the stretch values are not taken in consideration

It doesn't matter which values I put, they are not used in the actual window.