As elaborated in D19873 the vertical spacing between the master form layout and a specific wallpaper plugin was always too big. By setting the main column layout's spacing the 0 we avoid the added vertical space.
One step closer to perfection!
I think it's the correct solution as well. Maybe to elaborate better for the other reviewers:
We have 2 Kirigami form layouts within a column layout. The Kirigami layouts have their own spacing. However, the column layout adds unwanted spacing between the two form layouts because by default it's spacing is set to 5. To have the two form layouts seamlessly join with one another, we then just need to remove the column layout's spacing; the form layouts' spacing will suffice.