Order posts by custom date field

I would like the serach results to be ordered by a custom date field (auction_date_new). I use this same field to order the posts on archive pages. BUt it’s not working for the search page.

I have tried the filter:

add_filter( 'relevanssi_modify_wp_query', 'rlv_desc_date' );
function rlv_desc_date( $query ) {
	$query->set( 'meta_key', 'auction_date_new' );
    $query->set( 'order', 'DESC' );
	$query->set( 'orderby', 'meta_value_date' );
    return $query;
}

and variations of it. It changes the order, but seemingly randomly. It’s not in order of date (newest to oldest).