Session values getting nulled

Hey guys, I have a problem with session variables getting nulled on certain curcumstances. If a user logs into the clients single sign on solution he gets redirected to the Laravel application. With this initial request a special header will be sent. I need to retrieve the value from the header to be able to get user details from the database. I planned to retrieve the header in a Middleware (added to the web middleware group), get the details from the database and write the details into a session variable.

This approach works for the initial request, but after that, all requests will result in nulled session values.

<?php

namespace App\Http\Middleware;

use Closure;
use App\Models\UserDetail;

class CheckHeaders
{
    public function handle($request, Closure $next)
    {
        if ($request->headers->has('useridentifier')) {
            $userident = $request->header('useridentifier');
            $user = UserDetail::where('useridentifier', $userident)->first();
            session([
                'user' => [
                    'fn' => $user->firstname,
                    'ln' => $user->lastname,
                    'org' => $user->organisation,
                ]
            ]);
            return $next($request);
        }

        return abort(410);
    }
}

The result of the first request is:

'user' => [
                'fn' => 'jane',
                'ln' => 'doe',
                'org' => 'abc'
 ]

The result of every following request is always:

'user' => [
                'fn' => null,
                'ln' => null,
                'org' => null
]

What am I missing?