buffer creates a MultiPolygon instead of a Polygon

[flodaniel]

Using @turf/turf 6.5.0

Given a linestring.json and a buffer size of 500 meters, turf.buffer should (as per its types) generate a Polygon. However it creates a multipolygon.json

This is the code:

   turf.buffer(lineString, bufferSize, {
                            units: "meters",
                        });