Before a post is created there are no values in the ACF fields and they do not exist yet. The post meta values don’t exist until the first time a post is saved. As far as making the fields show up using the ACF to REST API, you’d probably need to talk to them about it. That is a 3rd party plugin and I think they’d be the best source of information.
Short of that you’d need to build something into your client side app that populates default values if none of returned for specific fields.
