"Cosmetic" line widths become very small line when printing PDFs

When drawing lines to a PDF via QPrinter, the width of "cosmetic" (zero-width) lines seems to have changed between Qt 5.9.7 and 5.12.2. In 5.9.7, the printed lines displayed the expected behavior, i.e. they always displayed as one pixel wide regardless of the zoom level in the PDF viewer. In 5.12.2 (also confirmed in 5.12.4), the lines end up being very thin "non-cosmetic" lines, which means that at "normal" zoom levels they appear faded out (sometimes drastically so) in most PDF viewers, and at very high zoom levels they become thicker than one pixel.

Compare the attached PDFs, which were generated using the same code compiled against two different versions of Qt. (Code is attached in pdf-line-weight.zip.)