Facebook Marketing API Leads returning empty data array

When calling https://graph.facebook.com/v12.0/<ad_id>/leads, I'm receiving an empty data array despite the fact that leads are showing in the Ads Manager dashboard for that ad.

{
  "data": [
  ]
}

To further prove that there should be leads, calling https://graph.facebook.com/v12.0/<ad_id>/insights?fields=ad_name,impressions,actions does return the correct number of leads:

{
  data: [
    {
      ...,
      "actions": [
        ...,
        {
          "action_type": "lead",
          "value": "2"
        }
      ]
    }
  ]
}

What is the problem here? How do I obtain the lead data from the API?