Flask session variable not persisting between Ajax requests

In my Flask app, I’m trying to display the progress of a backend task to the client. The client sends an initial request which begins the task. The task contains a loop, and as it iterates, it updates a session variable which contains the completion percentage. The client then regularly sends additional requests to retrieve the value of the session variable so it can be displayed.

The problem is that whenever the session variable is retrieved, it only contains its initial value of 0%. If I replace the session variable with a global variable, it works fine, but the app needs to support multiple simultaneous users, so I can’t use a global variable. My understanding is that session variables are meant for this exact purpose, to persist between requests within the same session, but I must be missing something, because it’s not doing so here.

Here’s a simplified version of my Flask code:

@app.route("/task")
def task():
    session["load_percent"] = 0
    percent = 0
    
    for i in range(10000000):
        #actual task performed here
            
        old_percent = percent
        percent = round(i / 10000000 * 100)
        if old_percent != percent:
            session["load_percent"] = percent
            print("Actual percentage: " + str(session["load_percent"]))
            #this prints the correct value of the session variable

    return "{}"

@app.route("/get_load_percent")
def get_load_percent():
    try:
        print("Retrieved percentage: " + str(session["load_percent"]))
        #this always prints 0
        return json.dumps({"load_percent": session["load_percent"]})
    except (KeyError):
        return "{}"

And my frontend JS:

function startTask() {
    $.ajax({
        type: "GET",
        url: $('body').data('task'),
        contentType: "application/json",
        dataType: 'json',
        success: function(response) {
            //respond to task completion here
        }
    });

    let percentageHTML = "";
    let percentageInterval = setInterval(function() {
        $.ajax({
            type: "GET",
            url: $('body').data('getloadpercent'),
            contentType: "application/json",
            dataType: 'json',
            success: function(response) {
                percentageHTML = response["load_percent"];
                console.log(percentageHTML);
                //always prints 0
            }
        })
    }, 100);
}

I’ve checked similar questions, and their solutions don’t work. The session variable does not exceed the size limitations, and the key is being set every time rather than having its value changed in place, so it doesn’t need to be marked as modified.