Redis Cluster for php session and ajax call not save session - php

Question

It was been a couple of days I've got an issue with php session (in cakePhp 4 project) with a redis cluster and ajax call that not save in session.
Use case :
I'm on a page with a session, session works perfectly. on this page there is à button that call an ajax action. On https://www.mywebsite.com ajax call https://www.mywebsite.com/ajaxAction (so no cross origin pb like I see a lot of time)
ajaxAction is very simple :
public function ajaxAction()
{
$this->disableAutoRender();
$treeId = (int) $this->request->getData('treeId');
var_dump($_SESSION['myArray']); // empty
$_SESSION['myArray'][$treeId] = true;
var_dump($_SESSION['myArray']); // array with the value
}
If I click to add an other $treeId in the session, the first debug is empty, and the second has only the new value.
When I check the session value in redis-cli, the value is not in the session.
If I put a sleep(1) after the $_SESSION['myArray'][$treeId] = true; the value is save in the session.
If I'm trying the same type of call with two simple php script (without cakePhp), it works perfectly....
Someone has a solution to understand the pb to find a solution ?
Is it from cakePhp ? Redis ? Ajax ?
Thanks for you're help
EDIT :
After another day of testing and searching a solution, the problem is only with cakephp. The only solution I've found is to make a usleep(100000); but it's not a solution of the problem for me....
EDIT 2 the return :
After analysing redis monitor, I could see that cakephp make two call to set the session during an ajax request. First save the new session, then save the old value of the session. It explains why with a sleep the new session is save after the old value.
Next step, undertand why cakephp make the two call of the session set (outside cakephp, there is only on cool in case of ajax request....)