Ajax is losing sessions

I upgraded my Symfony application from Symfony 4.0.7 to Symfony 4.1 and after that AJAX calls are losing sessions values.

I have about 6 ajax requests called at the same time. First of them is going fine but others are losing session values. It happened only after migration to Symfony 4.1 and only for AJAX calls. Any ideas?

edit: It happens only with ajax called at the same time. WHen I add eg 100 miliseconds delay between calling ajax then all works fine.

from Ajax is losing sessions