Add custom (ACF) fields to the WordPress Rest Api. https://since1979.dev/add-custom-acf-fields-to-the-wp-rest-api/

get-featured-image-api-field.php

<?php
/**
 * get_featured_image_api_field.
 *
 * Callback function for register_featured_image_api_field
 * Fetch and return the the featured image for a post.
 *
 * @see https://since1979.dev/add-custom-acf-fields-to-the-wp-rest-api/
 * @uses get_the_post_thumbnail_url() https://developer.wordpress.org/reference/functions/get_the_post_thumbnail_url/
 */
function get_featured_image_api_field($post)
{
    return get_the_post_thumbnail_url($post['id'], 'post-thumb-small');
}
?>