session.php

    // For anonymous users, we need to start session before storing in private
    // temp store, otherwise a new session_id will be created each page view
    // and the preious private store values will be inaccessible.
    if ($this->currentUser->isAnonymous() && !$this->request->getSession()->get('session_started')) {
      $this->request->getSession()->set('session_started', TRUE);
      $this->sessionManager->start();
    }