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.
constructor(parameters) {
    super(parameters);
    this.data.annotationType = AnnotationType.STRIKEOUT;
    const quadPoints = (this.data.quadPoints = getQuadPoints(
      parameters.dict,
      null
    ));
    if (quadPoints) {
      if (!this.appearance) {
        
        const strokeColor = this.color
          ? Array.from(this.color).map(c => c / 255)
          : [0, 0, 0];
        this._setDefaultAppearance({
          xref: parameters.xref,
          extra: "[] 0 d 1 w",
          strokeColor,
          pointsCallback: (buffer, points) => {
            buffer.push(
              `${(points[0].x + points[2].x) / 2}` +
                ` ${(points[0].y + points[2].y) / 2} m`
            );
            buffer.push(
              `${(points[1].x + points[3].x) / 2}` +
                ` ${(points[1].y + points[3].y) / 2} l`
            );
            buffer.push("S");
            return [points[0].x, points[1].x, points[3].y, points[1].y];