Automatically replace ampersand "&" with raw &

functions.php

<?php

// Add to your (child) theme's functions.php
// Make sure to re-index afterwards

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'YOUR_FACET_NAME' == $params['facet_name'] ) {
        $params['facet_display_value'] = str_replace( '&amp;', '&', $params['facet_display_value'] );
        $params['facet_display_value'] = str_replace( '&#x00026;', '&', $params['facet_display_value'] );
        $params['facet_display_value'] = str_replace( '&#38;', '&', $params['facet_display_value'] );
    }
    return $params;
}, 10, 2 );