Polyline doesn’t plot its lineweight

Today a colleague of mine could not plot a polyline at the desired lineweight. We tried different layer with the colour changed to see if was the plot style interferring. Tried turning off scale lineweights. Tried overriding the lineweight on the object itself.  Tried a new plot style.

After all these attempts it occured to me that the polyline thickness might have a impact on the lineweights and it does.

Altering the polyline global width to 0 meant the lineweight now plotted correctly.