index a serialized array of post IDs

<?php

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'YOUR_FACET_NAME_HERE' == $params['facet_name'] ) {
        $values = (array) $params['facet_value'];
        foreach ( $values as $post_id ) {
            $params['facet_value'] = (int) $post_id;
            $params['facet_display_value'] = get_the_title( $post_id );
            $class->insert( $params );
        }
        return false; // skip default indexing
    }
    return $params;
}, 10, 2 );