In the attached .ui file, the layoutSpacing for the vertical layout is set to 0 (zero). In the grid layout, the layoutHorizontalSpacing and layoutVerticalSpacing are left at their default values (-1). When previewing in designer, or running the uic-generated code, you can clearly see that the horizontal and vertical spacing in the grid layout are also set to zero.

This is not something I would expect at all.

When changing the layoutSpacing in the vertical layout to, say, 100, the same happens: h/v spacing in the grid is also 100. Changing the spacing of the grid layout to, say, 3, gives the expected result: no more propagation.