The GeoJSON spec states that a polygon's LinearRings should be closed. The geoCircle function does not necessarily produce a closed LinearRing (first element does not equal the last element).

Test script:

geoCircle().center([-74.006, 40.7128]).radius(8.993216059187306)()

First element:

[
  -85.42210701073721,
  38.73562276218336,
]

Last element:

[
  -85.42210701073715,
  38.73562276218316,
]