How is clicking 'update' on a post different from programatically creating posts?

hoping for some advice.

I'm programmatically inserting a large number of posts into wordpress from a JSON feed. The wp_insert_post function is working brilliantly and the posts are created, along with correctly populated Advanced Custom Fields meta data.

We have a strange issue by which until we manually click "update" on a single post the custom fields aren't available using a JSON API plugin.

I've tried updating all via the bulk editor, as well as calling wp_update_post after the JSON import. It's as if the act of clicking "update" on a single post saves the post in a different fashion.

Can anyone advise why this would be the case? Any advice or pointing in the right direction would be greatly appreciated!