WP_Query ignoring tax_query when is_singular

You can always use posts_clauses filter and add processing for tax_query in there (you don’t even need to write your own SQL):

function filter_single_with_tax_queries( $clauses, $query ) {
    global $wpdb;

    if ( $query->is_singular ) {  
        $query->parse_tax_query( $query->query_vars );
        $tax_clauses = $query->tax_query->get_sql( $wpdb->posts, 'ID' );
        $clauses['join'] .= $tax_clauses['join'];
        $clauses['where'] .= $tax_clauses['where'];
    }
    return $parts;
}
add_action( 'posts_clauses', 'filter_single_with_tax_queries', 10, 2 );