Hello LaurenBoyd,
Indeed the "outFields" are configured:
outFields: ["*"]
I also tried indicating them one by one, it doesn't change anything!
What intrigues me is that I could get the "Name" attribute on the buttons, when I was working on my project a few months ago, and with console.log I could see all my attributes, but not anymore. Is there a bug on Arcgis Online?
  require([
    "esri/Map",
    "esri/Basemap",
    "esri/views/MapView",	
    "esri/layers/FeatureLayer",
    "esri/layers/Layer",
	"esri/geometry/Extent",
    "esri/widgets/Legend",
    "esri/widgets/LayerList",
    "esri/widgets/Expand",
    "esri/widgets/Feature",
    "esri/Graphic",
	"esri/layers/GraphicsLayer",
	"esri/widgets/Home",
    "esri/PopupTemplate",
    "esri/widgets/ScaleBar",
	"esri/geometry/Point",
    "esri/symbols/SimpleMarkerSymbol",
    "esri/Color",
    "dojo/parser",
	"dojo/text!./data/tree.nwk",
    "dojo/domReady!"
  ], function (
    Map,
    Basemap,
    MapView,
    FeatureLayer,
    Layer,
	Extent,
    Legend,
    LayerList,
    Expand,
    Feature,
    Graphic,
	GraphicsLayer,
	Home,
    PopupTemplate,
    ScaleBar,
	Point,
	SimpleMarkerSymbol,
	Color,
	parser,
	newickSource
  ) {
    Layer.fromPortalItem({
      portalItem: {
        id: "a3fc6bbaaec642ce81e46711e08df208"
      },
      outFields: ["*"],
	  visible: true
    }).then(function (groupLayer) {
      const basemap = new Basemap({
        portalItem: {
          id: "00c8181753cd4673810a1ede1f52a922"
        }
      });
      const map = new Map({
        basemap: basemap,
        layers: groupLayer
      });
	  
      let mainExtent = new Extent({
        xmin: 30,
        ymin: 16,
        xmax: 60,
        ymax: 36,
        spatialReference: {
          wkid: 4326
        }
     });	  
   
      const view = new MapView({
        container: "viewDiv",
        map: map,
        center: [45.0, 26.0],
	    zoom: 4,
        popup: {
          highlightEnabled: false,
          autoOpenEnabled: false
        }
      });
      
	  const legend = new Legend({ view: view });
      const legendExpand = new Expand({
        expandTooltip: "Show Legend",
        expanded: false,
        view: view,
        content: legend
      });
      const layerList = new LayerList({ view: view });
      const scaleBar = new ScaleBar({ view: view, unit: "dual" });
	  const homeWidget = new Home({view: view});
  
      view.ui.add(layerList, "top-right");
      view.ui.add(legendExpand, "top-right");
      view.ui.add(scaleBar, { position: "bottom-left" });
	  view.ui.add(homeWidget, "top-left");
	  view.ui.add("logoDiv", "bottom-right");
	  
	  // Récupérer le bouton
       const toggleButton = document.getElementById("logoDiv");
      // Ajouter un gestionnaire d'événements pour le clic sur le bouton
       toggleButton.addEventListener("click", toggleLines);