If you increase line width to 2 pixels or more, line styles (dotted, dashes) will be visible. However, for 1 pixel width, separations between line segments, supposed to be in background color, are halfway between foreground and background colors, so at 100%, line looks almost solid.
(Paint.net v4.0.5, Win7x64 SP1, Hardware acceleration on, DPI 96.0, NVidia GTX 750)