What may cause inability to modify session in PHP?

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PHP

session

The objective is to delete id stored in session after logout.

Note:

1. session_start() has been called at the start of the request
2. Error reporting is turned on

What I have tried:

After using the usual unset, if I var_dump, it appears to have truly removed the id, yet the same id is available on subsequent requests to other routes. This doesn't apply to unset alone, as I have observed when inserting on that request too

public function signout () {

    unset($_SESSION['login_id'] );
    $_SESSION['jhg'] = 6778; // on next request to another route, this is nowhere to be found, while the login_id remains hail and hearty
    return [];
}

The only way I was able to successfully clear the value was with this hack

$eds = array_filter($_SESSION, function ($k) {
	return $k != 'login_id';
}, ARRAY_FILTER_USE_KEY);

$_SESSION = []; session_destroy(); 
session_start(); $_SESSION = $eds;

This works because I realized destroying the entire session is persisted across requests. That's the only operation on the superglobal that works on this particular request/route. Altering the session elsewhere works just as the docs say. What could be going on?