B2DLineJoin::Middle, drawPolyLine etc

What is B2DLineJoin::Middle supposed to be ?

I see that the vcl pdf, windows and quartz backends treat ::Middle the
same as ::Mitre (so I made the cairo one do the same)

On the the other hand I see cppcanvas...setupStrokeAttributes and
filter...ImplWriteLineInfo would default ::Middle to the same as ::NONE

while

cui...SvxLineTabPage::Reset and
svx...LinePropertyPanelBase::updateLineJoint treat LineJoin_MIDDLE the
same as ROUND but
filter...EscherPropertyContainer::CreateLineProperties and
oox...DrawingML::WriteOutline the same as BEVEL

Is ::Middle a "mistake" and we should just treat it as ::Mitre
everywhere ? Especially as that's what it turns into for the windows
vcl drawPolyLine. Or is there some subtlety here ?

C.