Always show featured (_featured) items first

test.php

<?php

// Add the following to your (child) theme's functions.php
add_filter( 'facetwp_filtered_post_ids', function( $post_ids, $class ) {
    global $wpdb;

    // Lookup featured post IDs
    $sql = "
    SELECT post_id
    FROM {$wpdb->prefix}postmeta
    WHERE meta_key = '_featured' AND meta_value = '1' AND post_id IN (" . implode( ',', $post_ids ) . ")";
    $results = $wpdb->get_col( $sql );

    // If a featured post, remove it from $post_ids and add it to $matches
    // We loop through $post_ids to preserve featured order
    $matches = array();
    foreach ( $post_ids as $key => $post_id ) {
        if ( in_array( $post_id, $results ) ) {
            $matches[] = $post_id;
            unset( $post_ids[ $key ] );
        }
    }

    // Featured first, then default sort
    $post_ids = array_merge( $matches, $post_ids );

    return $post_ids;
}, 15, 2 );