Menu separators missing in QGtkStyle with "Dust" or "Dust Sand" controls selected

As seen in the attached screenshot, the menu separator is missing when the Dust controls are selected, while they are visible as a single line in GTK menus.

Note that the line is entirely at the bottom of the space reserved for the separator (which seems to be only two pixels, too).

(btw, the icons are blurry because I'm using an old Qt version at the moment, this issue is only about the missing separator line, which also affects the latest Qt)