Teams doesn't display hero image

I've been trying to send cards to Teams channels through a connector. The cards do appear but any hero image I try to send isn't displayed. The Message Card Playground displays the image correctly but it simply doesn't show up in Teams. I have no issues with the "normal" images.

Here is the JSON that I'm sending through the webhook, but I've also tried the Twitter - Hero image sample card and it doesn't work either :

{
  "@type": "MessageCard",
  "@context": "http://schema.org/extensions",
  "summary": "Hero card testing",
  "title": null,
  "text": null,
  "themeColor": "19b5fe",
  "sections": [
    {
      "title": null,
      "activityImage": "https://pbs.twimg.com/profile_images/862653089916096512/ljJwcmFp_bigger.jpg",
      "activityTitle": "Hero image card",
      "activitySubtitle": "This is a test",
      "facts": [],
      "text": null,
      "heroImage": {
        "image": "https://pbs.twimg.com/media/DFv74A0XkAEdwQ_.jpg"
      },
      "images": []
    }
  ]
}

Am I missing something ?