Symfony 6, not able to read legacy session – Blancer.com Tutorials and projects

I have a legacy API application which makes traditional session_start(); and fills data into $_SESSION.
Now I am doing a v2 API with Symfony 6.4, in the same environment, and I want to read data form $_SESSION.

I found Integrating with Legacy Applications.
I don’t know if the legacy application has a mentioned own PHP save handler, I just tried all possible configuration combinations.

framework:
    session:
        storage_factory_id: session.storage.factory.php_bridge

I removed the var/cache & warmed it up.

Result:

$_SESSION: undefined
$this->requestStack->getSession(): Gives a complicated object tree, without legacy session data, but weird Symfony keys with empty arrays _sf2_attributes, _symfony_flashes.
$this->requestStack->getSession()->get('keyInLegacySession'): null

framework:
    session:
        storage_factory_id: session.storage.factory.php_bridge
        handler_id: ~

I removed the var/cache & warmed it up.

Result: Same as above.

framework:
    session:
        storage_factory_id: session.storage.factory.php_bridge
        handler_id: session.handler.native_file

I removed the var/cache & warmed it up.

Result: Same as above.

I tried to debug as explained in https://stackoverflow.com/a/44145831/4840661, bug legacy and Symfony give the same ini settings.

Any ideas how to read legacy session data in my case with Symfony?