sharepoint online – Default property values not working in SPFx web part


I have 3 web part properties set up in my spfx web part. However it is not showing the default values I have set

{
  "$schema": "https://developer.microsoft.com/json-schemas/spfx/client-side-web-part-manifest.schema.json",
  "id": "d1587168-2126-44f1-bc7d-cfa4cc5108ed",
  "alias": "MainArticlesWebPart",
  "componentType": "WebPart",

  // The "*" signifies that the version should be taken from the package.json
  "version": "*",
  "manifestVersion": 2,

  // If true, the component can only be installed on sites where Custom Script is allowed.
  // Components that allow authors to embed arbitrary script code should set this to true.
  // https://support.office.com/en-us/article/Turn-scripting-capabilities-on-or-off-1f2c515f-5d7e-448a-9fd7-835da935584f
  "requiresCustomScript": false,
  "supportedHosts": ("SharePointWebPart"),

  "preconfiguredEntries": ({
    "groupId": "5c03119e-3074-46fd-976b-c60198311f70", // Other
    "group": { "default": "Other" },
    "title": { "default": "Main Articles" },
    "description": { "default": "Main Articles description" },
    "officeFabricIconFontName": "Page",
    "properties": {
      "description": "Show the main articles.",
      "list_title": "Main Articles",
      "site_url": ""
    }
  })
}

and in the .ts file

  protected getPropertyPaneConfiguration(): IPropertyPaneConfiguration {
    return {
      pages: (
        {
          header: {
            description: strings.PropertyPaneDescription
          },
          groups: (
            {
              groupName: strings.BasicGroupName,
              groupFields: (
                PropertyPaneTextField('description', {
                  label: strings.DescriptionFieldLabel
                }),
                PropertyPaneTextField('list_title', {
                  label: strings.ListTitleFieldLabel
                }),
                PropertyPaneTextField('site_url', {
                  label: strings.SiteURLFieldLabel
                })
              )
            }
          )
        }
      )
    };
  }

It shows as

enter image description here

I can’t see what’s wrong. Does anyone know?