I'm trying to set a custom trial period when creating subscription using API (POST /subscriptions)

This is my payload:

1.             'customer_id' => 'xxxxxxx',
               'plan' => [
                   'plan_code' => 'xxxxx',
                   'setup_fee' => 120,
                   'trial_days' => 15
               ],
               'auto_collect' => false,

Subscription is created, but last billing date is set to today, instead of today+15 days:

1.   "created_at" => "2021-03-14"
     "activated_at" => "2021-03-14"
     "current_term_starts_at" => "2021-03-14"
     "current_term_ends_at" => "2021-04-14"
     "last_billing_at" => "2021-03-14"
     "next_billing_at" => "2021-04-14"
   

Also invoice is created and includes both setup fee and full price of the plan.

Expected behavior would be:

- subscription is created

- invoice for setup fee only should be generated

- after trial period expires (2021-03-14 + 15 days), invoice is generated for full plan price

- next billing periods will be counted from the end of trial period (e.g. 2021-03-29, 2021-04-29, etc.)