Floors are unique in that Revit will show them even if they are below the view range settings of the view (floor plan). They'll be visible until they exceed 4' (1200mm) below the level. Parts that are created from floors won't. In fact parts created from a floor that is at the level of the view (in other words, normal) won't show up either.
It is necessary to adjust the View Range to permit the parts to be visible. It would be more consistent with floors if their parts assumed the same quirk capability.