$session->isActive is just wrapper around session_status(), could be a something small are you trying to start a session yourself else where in your application, may be check for possible session_start() function calls.
Also you don’t to need to start a session manually yii will start a session the moment the session component is initialized.
yes exactly you don’t need to open the session and fiddle with lower level api unless really necessary, just use the higher level api’s provided. here is an example
// to set a value
Yii::$app->session->set('foo', 'bar');
// or get a value
Yii::$app->session->get('foo');
both set, get call open internally so you use that.