Background dissapears when generating photo avatar via API

Hi, I am doing an API call to generate a video from a photo avatar uploaded in my media.

The problem is that Heygen is making the background transparent, which is an issue I had never seen before.
You can see it here: https://app.heygen.com/videos/6e9a86b6418244d5ae64cfd3bd55abad (Video ID 6e9a86b6418244d5ae64cfd3bd55abad).

Here's the POST URL I am using: https://api.heygen.com/v2/video/av4/generate

And here's my JSON request:
{
"video_orientation": "portrait",
"image_key": "image/a36f0a53d6a040df81ecceeb995f8919/original",
"video_title": "[title]",
"script": "[script]",
"voice_id": "1600368e5cbd46cb81ab20f511caebee"
}

How can I prevent it from making the background transparent? That does not happen with other video photo avatars I am creating from uploaded media.

Thanks!