Spark not honouring the trial period for team products

I am testing a new implementation using Spark. A new user registers, setting up a team. There is a 30 day trial period. Then they subscribe to a plan, but they are billed immediately and the trial period is deleted. What am I doing wrong? Surely the subscription should commence at the end of the trial period?

The SparkServiceProvider.booted() method is:

    public function booted()
    {
        Spark::noCardUpFront()->teamTrialDays(30);
        Spark::collectBillingAddress();

        Spark::freeTeamPlan()
            ->maxTeamMembers(1)
            ->features([
                'First','Second','Third'
            ]);

        Spark::teamPlan('Monthly', 'plan_GsBB7yph5yJ4BJ')
            ->price(18)
            ->features([
                'First','Second','Third'
            ]);

       ...
    }

In Stripe, the plan has Trial Period 30 days, Interval Monthly and Price per unit £18.00.

After registration, the team record in the Laravel database has:

SELECT id, name, current_billing_plan, card_last_four, trial_ends_at FROM vapor.teams;
id, name, current_billing_plan, card_last_four, trial_ends_at
'1', 'Test Account', NULL, NULL, '2020-04-07 17:57:42'

When the user subscribes, various HTTP calls are made to Stripe to set up the customer and payment method. The subscription is set up with POST /v1/customers/cus_GsDvWlIR77e9VM/subscriptions with the following payload:

{
  "expand": {
    "0": "latest_invoice.payment_intent"
  },
  "plan": "plan_GsBB7yph5yJ4BJ",
  "quantity": "1",
  "trial_end": "now",
  "off_session": "true"
}

Why is Laravel sending "trial_end": "now" when the database has trial_ends_at set to 2020-04-07 17:57:42?

After the subscription, the teams record now reads:

id, name, current_billing_plan, card_last_four, trial_ends_at
'1', 'Test Account', 'plan_GsBB7yph5yJ4BJ', '4444', NULL