QTextFrameFormat::BorderStyle_Double rendered wrong

Create a QTextFrame with border style = QTextFrameFormat::BorderStyle_Double.

Expected to see:
A border composed of two concentric rectangles.

Got instead:
A border drawn with segments of double lines, but with incorrectly drawn corners.

It appears that the frames are rendered in the following sequence: 1) left end, 2) top and bottom butted to the left, 3) right end fit between the top and bottom. This also causes problems, but not as noticeable for these other border styles:

QTextFrameFormat::BorderStyle_Groove
QTextFrameFormat::BorderStyle_Ridge