PostGIS ST_Area unexpectedly returns 0

Can someone help me understand why the following ST_Area query returns 0:

SELECT ST_Area(ST_SetSRID(ST_GeomFromGeoJSON('{"type":"Polygon","coordinates":[[[-180.0,0],[180,0],[180,80],[-180,80],[-180,0]]]}'), 4326)::geography);
 st_area
---------
       0
(1 row)

but changing the coordinates slightly gives a reasonably large looking value for area.

af=> SELECT ST_Area(ST_SetSRID(ST_GeomFromGeoJSON('{"type":"Polygon","coordinates":[[[-179.99,0.01],[179.99,0.01],[179.99,79.99],[-179.99,79.99],[-179.99,0.01]]]}'), 4326)::geography);
     st_area
------------------
 13948451441.7438

For reference, this is my PostGIS version

SELECT PostGIS_Version();
            postgis_version
---------------------------------------
 2.5 USE_GEOS=1 USE_PROJ=1 USE_STATS=1

Follow up question: why are these areas different than the ones I see on geojson.io? Do I have a the wrong CRS or some other problem?

The docs say "by default area is determined on a spheroid with units in square meters" https://postgis.net/docs/ST_Area.html

If I put the same coordinates into geojson.io as feature collections, I see the following areas:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "Polygon",
        "coordinates": [[[-180,0],[180,0],[180,80],[-180,80],[-180,0]]]
      }
    }
  ]
}

gives an area of 251720748408616.50 Sq. Meters

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "Polygon",
        "coordinates": [[[-179.99,0.01],[179.99,0.01],[179.99,79.99],[-179.99,79.99],[-179.99,0.01]]]
      }
    }
  ]
}

gives an area of 251654405021681.66 Sq. Meters