When performing the search ACF uses the standard WP search feature.
You would need to add a filter https://www.advancedcustomfields.com/resources/acf-fields-relationship-query/
In this filter you would add a second filter following this https://stackoverflow.com/questions/9468804/make-wordpress-search-only-in-post-title
add_filter('acf/fields/relationship/query/name={YOUR FIELD NAME}', 'my_acf_fields_relationship_query', 10, 3);
function my_acf_fields_relationship_query( $args, $field, $post_id ) {
add_filter('posts_search', '__search_by_title_only', 500, 2);
return $args;
}
function __search_by_title_only($search, &$wp_query) {
// code from stack
}
