Could not send multiple response

[haward]

let response = {
'line': {
'type': 'image',
'originalContentUrl': 'https://vision.riverplus.com/wp-content/uploads/2020/01/qrcode.jpg',
'previewImageUrl': 'https://vision.riverplus.com/wp-content/uploads/2020/01/qrcode.jpg'
},
"facebook": {
"attachment": {
"type": "template",
"payload": {
"template_type": "generic",
"elements": [
{
"title": "Hello world",
"image_url": "https://vision.riverplus.com/wp-content/uploads/2020/01/qrcode.jpg",
"subtitle": "sub title text",
}
]
}
}
}
}
agent.add(
new Payload(agent.UNSPECIFIED, response, {
rawPayload: true,
sendAsMessage: true,
})
);
agent.add("Select one");
This code will send only the text "Select one"

Need to change parameter agent.UNSPECIFIED to specific channel for example
agent.add(
new Payload('FACEBOOK', response, {
rawPayload: true,
sendAsMessage: true,
})
);
agent.add("Select one");

This code is working.

Question is. Why it doesn't work when using an agent.UNSPECIFIED?