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.
if (data.fieldType === "Sig") {
      data.fieldValue = null;
      this.setFlags(AnnotationFlag.HIDDEN);
      data.hidden = true;
    }
  }
  /**
   * Construct the (fully qualified) field name from the (partial) field
   * names of the field and its ancestors.
   *
   * @private
   * @memberof WidgetAnnotation
   * @param {Dict} dict - Complete widget annotation dictionary
   * @returns {string}
   */
  _constructFieldName(dict) {
    
    
    if (!dict.has("T") && !dict.has("Parent")) {
      warn("Unknown field name, falling back to empty field name.");
      return "";
    }
    
    if (!dict.has("Parent")) {
      return stringToPDFString(dict.get("T"));
    }