Checking to see if user has subscriptions causes test to fail unexpectedly [Laravel Cashier]

I've created a method that processes user checkouts using cashier:

CheckoutController.php (working)

public function processCheckout(Request $request)
{
    $plan = Plan::findOrFail($request->billing_plan_id);

    try {
        $user = auth()->user()->newSubscription($plan->name, $plan->stripe_plan_id)->create($request->payment_method['id']);

        return response([], 201);
    } catch (\Exception $e) {
        return response([], 500);
    }
}

SubscriptionsTest

use RefreshDatabase;

protected $monthlyPlan;
protected $yearlyPlan;

protected function setUp(): void
{
    parent::setUp();

    $this->monthlyPlan = factory('App\Plan')->create(['name' => 'Monthly', 'stripe_plan_id' => 'plan_thing']); 
    $this->yearlyPlan = factory('App\Plan')->create(['name' => 'Yearly', 'stripe_plan_id' => 'plan_thing2']);      
}

/** @test */
public function a_user_can_subscribe_to_a_plan()
{
    $this->withoutExceptionHandling();

    $this->actingAs($user = factory('App\User')->create());

    \Stripe\Stripe::setApiKey(\Config::get('services.stripe.secret'));
    $payment_method = \Stripe\PaymentMethod::create([
        'type' => 'card',
        'card' => [
            'number' => '4242424242424242',
            'exp_month' => 5,
            'exp_year' => 2021,
            'cvc' => '314',
        ],
    ]);

    $response = $this->json('POST', '/api/checkout', ['billing_plan_id'=> 1, 'payment_method' => $payment_method ] )
        ->assertStatus(201);

    $this->assertTrue($user->subscribed('Monthly'));
}

Everything works great until I need to check to see if a user is already subscribed, and if they aren't create a subscription auth()->user()->newSubscription(). This is to prevent duplicate subscriptions.

CheckoutController (Not working)

public function processCheckout(Request $request)
{
    
    try {
        $plan = Plan::findOrFail($request->billing_plan_id);
        $user = $user = auth()->user();
        
        //If user has no subscriptions subscribe them to new plan
        if($user->subscriptions->count() === 0){

            $user->newSubscription($plan->name, $plan->stripe_plan_id)->create($request->payment_method['id']);

            //dd($user->subscribed('Monthly'));

            return response([], 201);
        }
    } catch (\Exception $e) {
        return response([], 500);
    }
}

Test:

Tests\Feature\SubscriptionsPlansTest::a_user_can_subscribe_to_a_plan Failed asserting that false is true.

As you can see I've //dd($user->subscribed('Monthly')); in the controller method. This returns false. dd($user->subscriptions->toArray()); also returns an empty array.

I'm unsure what is going on here and why dd($user->subscribed('Monthly')); is returning false. I'm pretty sure this works because the front end is working. What's going on?