Qt5 breaks rendering to pdf on Windows, for opacity and transparent QGradient

When I render a scene to a QPrinter set to PdfFormat, my QGraphicsItem subclass is rendered wrong on two counts:

1. opacity appears to be 1.0, no matter what I set it to.
2. With a QGradient, when I use setColorAt() to set the color to Qt::transparent, it uses black instead.

Rendering the scene to a QGraphicsView works correctly. But my users need to output to pdf. Rendering to a pdf does not have these issues in Qt 4.8, so it seems like a regression in Qt 5 or I'm missing something I should be doing in Qt 5.

For an sscce, see https://forum.qt.io/topic/71959/qt5-breaks-rendering-to-pdf-on-windows