sourceBuffer.mode = "sequence" doesn't work - Javascript

Question

I try to append buffer to sourceBuffer continuously for video. But when buffer appened then current scene pass to new appended one. I tried to fix it with timestampOffset but if there will a little gap then player stacks. After many research I figured out there is sourceBuffer.mode = "sequence" choice. But it didn't work for me. What may be the problem at my code?

const url_video = "/result_chunks/chunks1.mp4";
const url_video2 = "/result_chunks/chunks2.mp4";

const remoteCodec = `video/mp4; codecs="avc1.4d4020,mp4a.40.2"`;

const videoEl = document.querySelector("video");

const ms = new MediaSource();
videoEl.src = URL.createObjectURL(ms);

ms.addEventListener("sourceopen", function () {
    const sourceBuffer = ms.addSourceBuffer(remoteCodec);
    sourceBuffer.mode = "sequence";
    downloadChunks(sourceBuffer);
});

function downloadChunks(sourceBuffer) {
    fetch(url_video)
        .then((res) => res.arrayBuffer())
        .then((buffer) => {
            sourceBuffer.appendBuffer(buffer);
            sourceBuffer.onupdateend = () => {
                sourceBuffer.onupdateend = null;
                fetch(url_video2)
                    .then((res) => res.arrayBuffer())
                    .then((buffer) => {
                        sourceBuffer.appendBuffer(buffer);
                    });
            };
        });
}