Cura oddly slices thin walls

I want to print a small box (26x26x16 mm) with walls as thin as 0.4 mm without the top wall. I thought it would be fine, because in my Cura profile line width is 0.4 mm. When I slice it, Cura doesn't slice the walls and displays a shadow where they should be in the layers preview. When I increase wall thickness of the model to 0.8 mm - still the same thing. When I increase it to 1.0 mm, Cura finally slices the walls, but for some reason it chooses to make them one line thick, which is 0.4 mm, which it absolutely didn't like before. That doesn't make sense to me. How can I print walls at least 0.8 mm and get correct slicing from Cura?