responseCookies is always empty! why ?

 Why responseCookies is empty ?

import trio
import httpx


async def main():
    async with httpx.AsyncClient(base_url='https://api.zyte.com/v1/', auth=('hidden', ''), timeout=None) as client:
        data = {
            'url': 'https://legacy.com',
            'httpResponseBody': True,
            "experimental": {
                "responseCookies": True
            }
        }
        r = await client.post('extract', json=data)
        print(r.json()['experimental'])


if __name__ == "__main__":
    trio.run(main)