Spurious bounding box edges drawn by QPainter::drawPolyLine for Windows printer when pen colour has alpha != 255

I'm drawing an irregular polyline with a 'highlighter pen' effect by using alpha blending. On screen and when printing in Linux, this works perfectly, but when printing in Windows (Win7 32-bit), one or more of the edges of the polyline bounding box are drawn as solid black lines. The thickness of the lines varies, and as does exactly which edges are drawn (it may be that all are drawn but some are zero pixels wide). If I set alpha=255 for the pen colour, the problem goes away again, but any other value (including zero, so the polyline itself doesn't show at all) brings it back.