Temporary fix: handle ACF date fields (YYYYMMDD)

<?php

function fwp_index_acf_date( $params, $class ) {
    if ( 'vandring_startdatum' == $params['facet_name'] ) {
        $raw = $params['facet_value'];
        if ( 8 == strlen( $raw ) ) {
            $date_fixed = substr( $raw, 0, 4 ) . '-' . substr( $raw, 4, 2 ) . '-' . substr( $raw, 6, 2 );
            $params['facet_value'] = $date_fixed;
            $params['facet_display_value'] = $date_fixed;
        }
    }
    return $params;
}
add_filter( 'facetwp_index_row', 'fwp_index_acf_date', 10, 2 );