QComboBox has a border that uses the application palette when the border is set in an applied stylesheet

When using a style sheet style for a QComboBox,
If the border property is set, then some pixels around the down arrow are drawn with one of the application palette colours, and these pixels don't seem to be controllable with any style. (In the demo program I have set the palette to green)

I would expect that a QComboBox can be fully styleable with style sheets. This does not seem to be the case when the border property is set.

Please see attached demo program and images.