Qt 5.9, QCP 2.0.1
With linestyle as SolidLine the Disc is solid; OK. If linestyle is DottedLine the Disc is a mess - as if the outline of thedisc were painted with the line style.
Aha! I've just changed miterPen to include SolidLine and added miterPen to the Disc draw routine and it works OK now.
So it is a bug I think.