I happened to content ACF support about this recently and got this response. I was specifically interested in updates via REST.
I am afraid the acf/save_post action hook in WordPress is intended to be triggered only when a post is saved or updated via the WordPress administration interface (the back-end), and it wouldn’t be triggered for updates made through the REST API or other programmatic means. This was because the acf/save_post hook was designed to be tied to the post editor’s actions.
If you want to perform additional actions when a post is created or updated through the REST API, you can use the rest_insert_post(rest_after_insert_{$post_type}) and rest_after_insert_post(rest_after_update_{$post_type}) hooks, which are specifically provided for handling REST API-related post actions.
