How to search for the title of a related custom post type in the WordPress admin using ACF Pro

acf_register_fields.php

<?php if( function_exists('acf_add_local_field_group') ):

acf_add_local_field_group(array (
	'key' => 'group_558c7f75de508',
	'title' => 'Stories',
	'fields' => array (
		array (
			'key' => 'field_558c7fe4df0bc',
			'label' => 'Select artist',
			'name' => 'select_artist',
			'type' => 'relationship',
			'instructions' => '',
			'required' => 0,
			'conditional_logic' => 0,
			'wrapper' => array (
				'width' => '',
				'class' => '',
				'id' => '',
			),
			'post_type' => array (
				0 => 'artist',
			),
			'taxonomy' => array (
			),
			'filters' => array (
				0 => 'search',
				1 => 'taxonomy',
			),
			'elements' => array (
				0 => 'featured_image',
			),
			'min' => '',
			'max' => 1,
			'return_format' => 'object',
		),
	),
	'location' => array (
		array (
			array (
				'param' => 'post_type',
				'operator' => '==',
				'value' => 'story',
			),
		),
	),
	'menu_order' => 0,
	'position' => 'normal',
	'style' => 'default',
	'label_placement' => 'top',
	'instruction_placement' => 'label',
	'hide_on_screen' => '',
));

endif;

wpsx_185734.php

<?php
/*
Plugin Name: WPSX 185734: Admin search ACF relationship
Plugin URI:  https://wordpress.stackexchange.com/questions/185734/admin-search-acf-relationship
Description: Demonstrates how to search for the title of a related custom post type in admin
Version:     1.0
Author:      Jan Beck
Author URI:  http://jancbeck.com
License:     GPL2
License URI: https://www.gnu.org/licenses/gpl-2.0.html
Text Domain: acf-relationship-search
*/

add_action( 'pre_get_posts', 'wpsx_185734_acf_search_relationship' );

function wpsx_185734_acf_search_relationship( $q ) {

	$screen = get_current_screen();
	$s = $q->get('s');
	$post_type = $q->get('post_type');

	// Be very selective when running code at the pre_get_posts hook
	if ( ! is_admin() || empty( $s ) || ( isset( $screen->post_type ) && 'story' != $screen->post_type ) || 'story' != $post_type ) {
		return;
	}
	
	// get all artists that match the search parameter $s
	$found_artists = get_posts( array('post_type' => 'artist', 'nopaging' => true, 's' => $s, 'fields' => 'ids') );
	
	// build a meta query to include all posts that contain the matching artist IDs in their custom fields
	$meta_query = array();
	foreach ( $found_artists as $artist_id ) {
		$meta_query[] = array(
			'key' => 'select_artist', // name of custom field
			'value' => '"' . intval($artist_id) . '"', // matches exactly "123", not just 123. This prevents a match for "1234"
			'compare' => 'LIKE'
		);
	}
	$q->set( 'meta_query', $meta_query );		
	$q->set( 's', '' ); // unset the original query parameter to avoid errors
}

add_action( 'init', 'wpsx_185734_register_customs' );

function wpsx_185734_register_customs() {
	$labels = array(
		'name'               => _x( 'Stories', 'post type general name', 'acf-relationship-search' ),
		'singular_name'      => _x( 'Story', 'post type singular name', 'acf-relationship-search' ),
		'menu_name'          => _x( 'Stories', 'admin menu', 'acf-relationship-search' ),
		'name_admin_bar'     => _x( 'Story', 'add new on admin bar', 'acf-relationship-search' ),
		'add_new'            => _x( 'Add New', 'story', 'acf-relationship-search' ),
		'add_new_item'       => __( 'Add New Story', 'acf-relationship-search' ),
		'new_item'           => __( 'New Story', 'acf-relationship-search' ),
		'edit_item'          => __( 'Edit Story', 'acf-relationship-search' ),
		'view_item'          => __( 'View Story', 'acf-relationship-search' ),
		'all_items'          => __( 'All Stories', 'acf-relationship-search' ),
		'search_items'       => __( 'Search Stories', 'acf-relationship-search' ),
		'parent_item_colon'  => __( 'Parent Stories:', 'acf-relationship-search' ),
		'not_found'          => __( 'No stories found.', 'acf-relationship-search' ),
		'not_found_in_trash' => __( 'No stories found in Trash.', 'acf-relationship-search' )
	);

	$args = array(
		'labels'             => $labels,
		'public'             => true,
		'publicly_queryable' => true,
		'show_ui'            => true,
		'show_in_menu'       => true,
		'query_var'          => true,
		'rewrite'            => array( 'slug' => 'story' ),
		'capability_type'    => 'post',
		'has_archive'        => true,
		'hierarchical'       => false,
		'menu_position'      => null,
		'supports'           => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
	);

	register_post_type( 'story', $args );

	$labels = array(
		'name'               => _x( 'Artists', 'post type general name', 'acf-relationship-search' ),
		'singular_name'      => _x( 'Artist', 'post type singular name', 'acf-relationship-search' ),
		'menu_name'          => _x( 'Artists', 'admin menu', 'acf-relationship-search' ),
		'name_admin_bar'     => _x( 'Artist', 'add new on admin bar', 'acf-relationship-search' ),
		'add_new'            => _x( 'Add New', 'artist', 'acf-relationship-search' ),
		'add_new_item'       => __( 'Add New Artist', 'acf-relationship-search' ),
		'new_item'           => __( 'New Artist', 'acf-relationship-search' ),
		'edit_item'          => __( 'Edit Artist', 'acf-relationship-search' ),
		'view_item'          => __( 'View Artist', 'acf-relationship-search' ),
		'all_items'          => __( 'All Artists', 'acf-relationship-search' ),
		'search_items'       => __( 'Search Artists', 'acf-relationship-search' ),
		'parent_item_colon'  => __( 'Parent Artists:', 'acf-relationship-search' ),
		'not_found'          => __( 'No artists found.', 'acf-relationship-search' ),
		'not_found_in_trash' => __( 'No artists found in Trash.', 'acf-relationship-search' )
	);

	$args = array(
		'labels'             => $labels,
		'public'             => true,
		'publicly_queryable' => true,
		'show_ui'            => true,
		'show_in_menu'       => true,
		'query_var'          => true,
		'rewrite'            => array( 'slug' => 'artist' ),
		'capability_type'    => 'post',
		'has_archive'        => true,
		'hierarchical'       => false,
		'menu_position'      => null,
		'supports'           => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
	);

	register_post_type( 'artist', $args );
}