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.
const dict = params.dict;
    this.setContents(dict.get("Contents"));
    this.setModificationDate(dict.get("M"));
    this.setFlags(dict.get("F"));
    this.setRectangle(dict.getArray("Rect"));
    this.setColor(dict.getArray("C"));
    this.setBorderStyle(dict);
    this.setAppearance(dict);
    this._streams = [];
    if (this.appearance) {
      this._streams.push(this.appearance);
    }
    
    this.data = {
      annotationFlags: this.flags,
      borderStyle: this.borderStyle,
      color: this.color,
      contents: this.contents,
      hasAppearance: !!this.appearance,
      id: params.id,
      modificationDate: this.modificationDate,
      rect: this.rectangle,
      subtype: params.subtype,
    };
    this._fallbackFontDict = null;
  }
  /**
   * @private
   */
  _hasFlag(flags, flag) {