Hi Elliot!
Thanks for the greatest plugin for WP and for your answer 🙂
var_dump() says that my $columns is NULL.
But I fixed my problem by this code in my functions.php:
add_action( 'user_search_columns', 'acf_name_search' );
function acf_name_search() {
$columns[] = "display_name";
return $columns;
}
Now it works great for me.