Why doesn't appendTimeStamp Yii2 work?

Question

Yii

Good afternoon.
Why does adding a timestamp to css files not work, but at the same time, to JS files it works.

class SiteAsset extends AssetBundle
{
    public $basePath = '@webroot';
    public $baseUrl = '@web/res/site';
    public $css = [
        'css/styles.css',
        'css/z-slidemenu.css',
        'css/z-selectric.css',
        'css/z-checkbox.css',
        'css/inline.css',

    ];
    public $js = [
        'js/bs-animation.js',
        'js/jquery.selectric.js',
        'js/scripts.js',
        'js/selectric.js',
        'js/slidemenu.js',
        'js/sliders.js',
        'js/jquery-cookie.js',
    ];

Adding a label is enabled in the config.
Thanks in advance.