为什么在几乎在执行ajax get请求的同时发布数据时，不会保存php会话数据？

Using a html element for search, my search allows GET and POST requests. I am sending the pressed keys using ajax GET for live search (timeout 750ms) and when the form is submitted, the string is POSTED.

The problem is that new session data is not saved when the ajax request and the form is submitted at the same time.

I found two solutions to this problem but they seem to mask the real problem. First solution is to stop the ajax request when enter is pressed using javascript if ((event.keyCode || event.which) == 13) and 2nd solution is calling session_write_close() and session_start() right after new session data is saved.

Can you explain why some session data is not saved (while others are properly saved during same request) when ajax GET is executed while a html form is in the middle of being posted or else explain the need for calling session_write_close and session_start to make sure session data is saved during critical operations like checking CSRF: generate new token if post is valid and save this in session?

Example code:

PHP Wrapper for storing new key:

public function setCsrfKey($property)
{
    $_SESSION['x']['CsrfKey'] = (string) $property; 
}

JS code:

$(searchBox).on("keyup", function(e){
    e.stopPropagation();
    clearTimeout(ajaxTimeOut);

    var keyPressed = e.keyCode || e.which;

    var string = $(this).val();

    if (string.length < 3) {
        queueList.clearQueue('ajaxCall');

        return;
    }

    queueList.queue('ajaxCall', function(){
        $.ajax({
            url: urlString,
            type: 'GET',
        }).done (function( data ) {
        }).fail (function(){
        });
    });

    ajaxTimeOut = setTimeout( function(){
        while (queueList.queue('ajaxCall').length > 1) {
            queueList.queue('ajaxCall').shift();
        }
        queueList.dequeue('ajaxCall');
    }, 750);

});