Hi,
Yes, isAjax() returns true.
In steps:
1. I create session in BaseController.
2. Display some view with jQuery.
3. This view sends Ajax in 10 secs interval.
4. Effect = every interval runs \CodeIgniter\Session\Session->setCookie method and then rewrite expiration time with new (mayby this is bug?).
Now I find solution for this, override this method by own: