create a campaign using a 'new builder' template

Question

I have a MailChimp Email Template that looks like the following (note the id 10564231)

I am creating a new campaign via the API

{
  "type": "regular",
  "recipients": {
    "list_id": "..."   
  },
  "settings": {
    "subject_line": "a test",
    "preview_text": "",
    "title": "",
    "from_name": "...",
    "reply_to": "...",    
    "template_id": 10564231
  },   
  "content_type": "template"
}

this works and the returned JSON looks similar to

{
...
    "settings" : {
    ...
    "template_id": 10564231
    }
}

however when you look at the campaign it does not use the template - instead it has some placeholder info that looks like this

It seems to have reverted to 'Classic builder' and the template is not being used.

Why is mailchimp not using the template?

As an aside, if you use a template that was created with the "Classic" builder everything works