Collapsed menu dropdown missing 1st level items

[ddt368]

When using the NavBar collapse (for small screen) to convert Nav inline $menuItems to drop down list, which works, the variable menu items for Signup and Login do not appear on the dropdown.
Test example from view/layouts/main.php -- all 8 items appear on the full bar but only 6, Who-Why, appear on the collapsed dropdown. I expect to see Signup and Login, or Logout, as well. I am reporting this as a potential bug as I am not seeing where the array would have changed.

$menuItems = [
['label' => 'Who', 'url' => ['/site/who']],
['label' => 'What', 'url' => ['/site/what']],
['label' => 'When', 'url' => ['/site/when']],
['label' => 'Where', 'url' => ['/site/where']],
['label' => 'How', 'url' => ['/site/how']],
['label' => 'Why', 'url' => ['/site/why']],
];
if (Yii::$app->user->isGuest) {
$menuItems[] = ['label' => 'Signup', 'url' => ['/site/signup']];
$menuItems[] = ['label' => 'Login', 'url' => ['/site/login']];
} else {
$menuItems[] = '

'
. Html::beginForm(['/site/logout'], 'post')
. Html::submitButton(
'Logout (' . Yii::$app->user->identity->username . ')',
['class' => 'btn btn-link logout']
)
. Html::endForm()
. '

';
}

Yii 2.024
PHP 7.0.33
Debian Stretch