Record and then download the video in mp4 format

Question

I'm recording the video and downloading it as an mp4. After downloading the video, It can not be seek forward(video remapping) in any player like if I want to seek the video to 10 second forward it's not working. I tried many different solutions but nothing works, what is wrong here?

const videoElem = document.getElementById('stream-elem')

var startBtn = document.getElementById('start-stream')
var endBtn = document.getElementById('stop-media')

var recorder;

const settings = {
video: true,
audio: true
}

startBtn.addEventListener('click', function (e) {
    navigator.mediaDevices.getUserMedia(settings).then((stream) => {
        console.log(stream);
        videoElem.srcObject = stream

        recorder = new MediaRecorder(stream)
        console.log(recorder);

        recorder.start();

        const blobContainer = [];

        recorder.ondataavailable = function (e) {
            
            blobContainer.push(e.data)
        }

        recorder.onerror = function (e) {
            return console.log(e.error || new Error(e.name));
        }


        recorder.onstop = function (e) {
            console.log(window.URL.createObjectURL(new Blob(blobContainer)));
            var newVideoEl = document.createElement('video')
            newVideoEl.height = '400'
            newVideoEl.width = '600'
            newVideoEl.autoplay = true
            newVideoEl.controls = true
            newVideoEl.innerHTML = `<source src="${window.URL.createObjectURL(new Blob(blobContainer))}"
             type="video/mp4">`
            document.body.removeChild(videoElem)
            document.body.insertBefore(newVideoEl, startBtn);

            var formdata = new FormData();
            formdata.append('blobFile', new Blob(blobContainer));


            fetch('uploader.php', {
                method: 'POST',
                body: formdata
            }).then(()=>{
                alert('streamed video file uploaded')
            })
        }

    })
    
})

   endBtn.addEventListener('click', function (e) {
        videoElem.pause();
        recorder.stop();
})