<?php
function bw_filter_the_title( $title, $post = 0 ) {
	$post = get_post( $post );
	if ( !is_admin() && function_exists( 'get_field' ) ) {
		$meta_title = get_field( 'custom_title', $post );
		if ( ! empty( $meta_title ) ) {
			$title = $meta_title;
		}
	}

	return $title;
}
add_filter( 'the_title', 'bw_filter_the_title', 10, 2 );