Laravel testing: get() method persists query string

Question

I'm running Laravel 5.6 and came across a weird issue while testing end points with query string.

Here's my test:

/**
 * @test
 */
public function returns_records_filtered_by_search()
{
    factory(NewsletterSubscriber::class)->states('verified')->create([
        'email' => 'jon@doe.com',
        'first_name' => 'Jon',
        'last_name' => 'Doe',
    ]);

    factory(NewsletterSubscriber::class)->states('verified')->create([
        'email' => 'jane@doe.com',
        'first_name' => 'Jane',
        'last_name' => 'Doe',
    ]);

    factory(NewsletterSubscriber::class)->states('verified')->create([
        'email' => 'jeremy@doe.com',
        'first_name' => 'Jeremy',
        'last_name' => 'Doe',
    ]);


    $response = $this->get(route('admin.newsletter_subscriber', ['q' => 'doe']));

    $response->assertStatus(Response::HTTP_OK)
    $response->assertSee('Subscribers (3)');

    $response->assertSee('jon@doe.com');
    $response->assertSee('jane@doe.com');
    $response->assertSee('jeremy@doe.com');


    $response = $this->get(route('admin.newsletter_subscriber', ['q' => 'jane']));

    $response->assertStatus(Response::HTTP_OK)
    $response->assertSee('Subscribers (1)');

    $response->assertDontSee('jon@doe.com');
    $response->assertSee('jane@doe.com');
    $response->assertDontSee('jeremy@doe.com');
}

Each of the 2 $this->get() calls seem to be calling the route with the same value for the query string parameter q = doe and hence the second set of assertions fails as it also returns 3 records.

When tested in browser it all works fine and records are filtered by the query string - it's just during tests - as if the call was ignoring parameters second time around.

route helper returns correct url with the query string.

My controller has something like this:

public function index(Request $request): View
{
    $query = NewsletterSubscriber::verified();

    if (!is_null($q = $request->get('q'))) {

        $query->where(function(Builder $query) use ($q) {
            $query->where('email', 'like', '%'.$q.'%')
                ->orWhere('first_name', 'like', '%'.$q.'%')
                ->orWhere('last_name', 'like', '%'.$q.'%');
        });

    }

    $subscribers = $query->orderBy('created_at', 'desc')->paginate(10);

    return $this->view('newsletter-subscriber.index')
                ->with('subscribers', $subscribers);
}

Any idea what might be causing it?