Openlayers geojson-vt : Why no features are generated?

I implemented openlayers geojson-vt example. I try to load a genuine geojson... I don't understand why no features are generated: sandbox demo

Where is the issue causing the difference with openlayers website example?

I try to generate features

Your data is in EPSG:2154. geojson-vt only supports data in EPSG:4326 so you must convert it

fetch(url)
  .then(function (response) {
    return response.json();
  })
  .then(function (json) {
    proj4.defs(
      'EPSG:2154',
      '+proj=lcc +lat_0=46.5 +lon_0=3 +lat_1=49 +lat_2=44 +x_0=700000 +y_0=6600000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs +type=crs',
    );
    register(proj4);
    const format = new GeoJSON();
    return format.writeFeaturesObject(format.readFeatures(json), {
      featureProjection: 'EPSG:2154',
      dataProjection: 'EPSG:4326',
    });
  })
  .then(function (json) {

Also your features do not have a COLOR property, so you should use a different style.