functions.php

<?php

add_filter( 'facetwp_indexer_row_data', function( $rows, $params ) {
    if ( 'locations' == $params['facet']['name'] ) {
        $post_id = (int) $params['defaults']['post_id'];
        $location = get_field( 'locations', $post_id ); // returns an post object
        $field_value = get_field( 'postcode', $location->ID );
        $new_row = $params['defaults'];
        $new_row['facet_value'] = $field_value; // value
        $new_row['facet_display_value'] = $field_value; // label
        $rows[] = $new_row;
    }
    return $rows;
}, 10, 2 );

LOL please solve my issue too.