Thanks to the support team, I got this to work. You don’t need to reference $post_id
at all. Here’s my code:
add_filter('acf/fields/post_object/query', 'prefix_only_published_posts', 10, 3);
/**
* Display only published posts
*/
function prefix_only_published_posts( $args, $field, $post_id ) {
$args['post_status'] = 'publish';
$args['orderby'] = 'date';
$args['order'] = 'DESC';
return $args;
}