I'm trying to get my head around this: I don't know why the twig debug isn't working:
Drupal version 8.2.6

Folder structure:

.

Settings.php

$settings['hash_salt'] = 'DEVELOPMENT_SALT';
$settings['update_free_access'] = FALSE;
$settings['file_public_base_url'] = 'http://localhost/files';
$settings['file_public_path'] = 'sites/default/files';
$settings['file_private_path'] = 'sites/default/private';
$settings['file_scan_ignore_directories'] = [
  'node_modules',
  'bower_components',
];
if (file_exists(__DIR__ . '/../development/settings.development.php'))     {
  include __DIR__ . '/../development/settings.development.php';
}

Here I include /../development/settings.development.php

assert_options(ASSERT_ACTIVE, TRUE);
\Drupal\Component\Assertion\Handle::register();

/**
 * Enable local development services.
 */
$settings['container_yamls'][] = __DIR__ . '/development.services.yml';

$databases['default']['default'] = array(
  'database' => 'dbname',
  'username' => 'dbusername',
  'password' => 'pw',
  'prefix' => '',
  'host' => '127.0.0.1',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
);

$settings['hash_salt'] = 'DEVELOPMENT';

$config['system.logging']['error_level'] = 'verbose';
$config['system.performance']['css']['preprocess'] = FALSE;
$config['system.performance']['js']['preprocess'] = FALSE;
$settings['cache']['bins']['render'] = 'cache.backend.null';
$settings['cache']['bins']['dynamic_page_cache'] =     'cache.backend.null';
$settings['extension_discovery_scan_tests'] = TRUE;
$settings['rebuild_access'] = TRUE;
$settings['skip_permissions_hardening'] = TRUE;

Here I also add development.services.yml, so:

parameters:
  http.response.debug_cacheability_headers: true
  twig.config:
    debug: true
    auto_reload: true
    cache: false
services:
  cache.backend.null:
    class: Drupal\Core\Cache\NullBackendFactory

Things I have checked/done:
Checked if all these files are loaded -> They are.
Checked the indentation of the development.services.yml -> Checks out.
Cleared all the different caches, rebuild them, ...
-> If I change a template, it changes, but no twig debugging.

What is odd: sites/default/files AND sites/default/private are not used by Drupal.