Magento 2 Multiselect Store View - Can't save multiple values

Question

I want to add a Multiselect under Stores -> Settings -> All Stores -> Store View

I've added a new Field:

\Test\StoreSwitcher\Block\Store.php

<?php

namespace Test\StoreSwitcher\Block;

class Store extends \Magento\Backend\Block\System\Store\Edit\Form\Store
{
    protected function _prepareStoreFieldset(\Magento\Framework\Data\Form $form) {
        .....
        $storeSwitcherIds = $storeModel->getSwitcherIds();
        $fieldset->addField(
            'store_switcher_ids',
            'multiselect',
            [
                'name' => 'switcher_ids',
                'label' => __('Store Switcher'),
                'value' => $storeSwitcherIds,
                'values' => $this->getStores(),
                'required' => false,
                'disabled' => $storeModel->isReadOnly()
            ]
        );
        .....

My Problem is, if I select multiple values, switcher_ids has only one value.

I've checked Post Request and the error is already here. switcher_ids[] should have two values.

Does somebody know what is wrong?