`data-tf-hidden` not working on live embeds

Using the new ‘live’ embed code I can’t seem to set hidden fields directly in the snippet.

I’m doing this:

<div
  data-tf-live="<embed-id>"
  data-tf-hidden="user_id=123"
></div>
<script src="//embed.typeform.com/next/embed.js"></script>

But when the page loads, the actual widget embed generated inside that div contains data-tf-hidden=“user_id=”, and the form submits with an empty value for that field.