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 newRef = xref.getNewRef();
    const AP = new Dict(xref);
    AP.set("N", newRef);
    const encrypt = xref.encrypt;
    let originalTransform = null;
    let newTransform = null;
    if (encrypt) {
      originalTransform = encrypt.createCipherTransform(
        this.ref.num,
        this.ref.gen
      );
      newTransform = encrypt.createCipherTransform(newRef.num, newRef.gen);
      appearance = newTransform.encryptString(appearance);
    }
    dict.set("V", isAscii(value) ? value : stringToUTF16BEString(value));
    dict.set("AP", AP);
    dict.set("M", `D:${getModificationDate()}`);
    const appearanceDict = new Dict(xref);
    appearanceDict.set("Length", appearance.length);
    appearanceDict.set("Subtype", Name.get("Form"));
    appearanceDict.set("Resources", this._getSaveFieldResources(xref));
    appearanceDict.set("BBox", bbox);
    const bufferOriginal = [`${this.ref.num} ${this.ref.gen} obj\n`];
    writeDict(dict, bufferOriginal, originalTransform);
    bufferOriginal.push("\nendobj\n");
    const bufferNew = [`${newRef.num} ${newRef.gen} obj\n`];
    writeDict(appearanceDict, bufferNew, newTransform);
    bufferNew.push(" stream\n");
    bufferNew.push(appearance);