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 (!isName(style)) {
      return;
    }
    switch (style.name) {
      case "S":
        this.style = AnnotationBorderStyleType.SOLID;
        break;
      case "D":
        this.style = AnnotationBorderStyleType.DASHED;
        break;
      case "B":
        this.style = AnnotationBorderStyleType.BEVELED;
        break;
      case "I":
        this.style = AnnotationBorderStyleType.INSET;
        break;
      case "U":
        this.style = AnnotationBorderStyleType.UNDERLINE;
        break;
      default:
        break;
    }
  }