Fix BBPress bug raised here https://bbpress.org/forums/topic/bbpress-breaks-wp_query-in-certain-cases/

4m-bbpress-fix.php

<?php
/**
 * Plugin Name: 4m - BBpress fix
 * Plugin URI: http://4mation.com.au
 * Description: Fix queries when user is not logged in.
 * Version: 1.0
 * Author: 4mation
 * Author URI: http://www.4mationtechnologies.com.au
 * License: A "Slug" license name e.g. GPL2
 */
// don't load directly
if ( !defined('ABSPATH') ) {
    die('-1');
}
Class FmBbpressFix {
    function __construct() {
        add_filter( 'bbp_exclude_forum_ids', array( __CLASS__, 'fixQuery' ), 10, 3 );
    }

    /**
     * Fix a bug from BBPress.
     * When a classic get_posts query is launched, BBP adds a meta_query with private forums ids that prevents the others results to be shown
     */
    public static function fixQuery( $retval, $forum_ids, $type = '' ) {
        if ( $type != 'meta_query' ) {
            return $retval;
        }

        if ( !is_array( $retval ) || !isset( $retval['compare'] ) || $retval['compare'] != 'NOT IN' ) {
            return $retval;
        }

        // Use not exists instead of not in which means that non logged in users will not see any forums instead of any private forums
        $retval['compare'] = 'NOT EXISTS';
        return $retval;
    }
}
new FmBbpressFix();