Cannot enable Wordpress Network - 'MULTISITE' already defined and false

In wp-config.php I have this code:

var_dump(defined('WP_ALLOW_MULTISITE'));  // false
var_dump(defined('MULTISITE'));           // true
var_dump(defined('SUBDOMAIN_INSTALL'));   // false

define('WP_ALLOW_MULTISITE', true );
var_dump(WP_ALLOW_MULTISITE);             // true
define('MULTISITE', true);
var_dump(MULTISITE);                      // false
define('SUBDOMAIN_INSTALL', true);
var_dump(SUBDOMAIN_INSTALL);              // true

MULTISITE constant is already defined with false before wp-confing.php. I searched with grep and it's defined only in ./wp-admin/includes/network.php as define('MULTISITE', true);.

How to make it true? I tried on local Laradock instance and shared hosting. In both cases result is the same.

EDIT (new info)

Results of running grep -Rn MULTISITE ./ | grep define | grep -v defined

./wp-config.php:118:define( 'WP_ALLOW_MULTISITE', true );
./wp-config.php:121:define('MULTISITE', true);
./wp-settings.php:125:  define( 'MULTISITE', false );
./wp-admin/includes/network.php:439:define('MULTISITE', true);

After enabling debugging in wp-config.php with define('WP_DEBUG', true); I am getting this notice in the admin area:

Notice: Constant MULTISITE already defined in /wordpress/wp-config.php on line 120

EDIT (solution)

My bad, I put define() calls below require_once for wp-settings.php in wp-config.