That is a rendering issue. 
The example you gave looks to be correctly rendered. The 'edge' of a park does not appear to be drawn - just the 'fill' .. see https://www.openstreetmap.org/way/388948281.

A 'national park' is tagged as a boundary and is drawn as a line around the outside - no fill. See https://www.openstreetmap.org/relation/6110851
