Webm duration broken

Question

I am recording screen with mediaRecorder in 10 second chunks of video.
Then, with node.js, we put those videos together in a single webm.

The problem is that we can't see the duration of the video when we play it.

We tried the package webm-duration-fix but I can only apply it to the video fragments, and when joining them it is broken

I'm using Javascript on the front and node js on the back. In the back I get the video snippet and then apply fs.appendFileSync

app.post('/upload/', async (req, res) => {
    let dir = './videos';
    let testeador = req.body.testeador;
    let cliente = req.body.cliente;
    let id = req.body.id;
    let fragmento = req.body.fragmento;
    let idVideo = generateRandomCode()


    res.send('Archivo subido')


    fs.mkdirSync(`${dir}/${cliente}/${testeador}`, {
        recursive: true
    });

    fs.appendFileSync(`${dir}/${cliente}/${testeador}/${testeador}.webm`, req.files.video.data, (err) => {
        if (err) throw err;
    });

})