In file https://github.com/mozilla/pdf.js/files/5249872/product1.pdf, the fifth text widget has a default appearance using font F1 and this one is neither in DR nor in AcroForm but in AP::N::Resources.
case "Tx":
            return new TextWidgetAnnotation(parameters);
          case "Btn":
            return new ButtonWidgetAnnotation(parameters);
          case "Ch":
            return new ChoiceWidgetAnnotation(parameters);
        }
        warn(
          'Unimplemented widget field type "' +
            fieldType +
            '", ' +
            "falling back to base field type."
        );
        return new WidgetAnnotation(parameters);
      case "Popup":
        return new PopupAnnotation(parameters);
      case "FreeText":
        return new FreeTextAnnotation(parameters);
      case "Line":
        return new LineAnnotation(parameters);
      case "Square":
        return new SquareAnnotation(parameters);
      case "Circle":
        return new CircleAnnotation(parameters);
      case "PolyLine":
        return new PolylineAnnotation(parameters);
      case "Polygon":
        return new PolygonAnnotation(parameters);
      case "Caret":
        return new CaretAnnotation(parameters);