Extbase MVC Framework

In my model I have an object "App" containing (mm) "Button" objects. Extbase seems to remove the relation to the Button in the App object when calling $this->buttonRepository->remove($button);

Here is my configuration, unnecessary parts stripped ([...]):

Objects:

class Tx_LalAppfactory_Domain_Model_App extends Tx_Extbase_DomainObject_AbstractEntity {

    [...]

    /**
     * @var Tx_Extbase_Persistence_ObjectStorage<Tx_LalAppfactory_Domain_Model_Button>
     * @lazy
     * @remove
     */
    protected $buttons;

    /**
     * Constructor. Initializes all Tx_Extbase_Persistence_ObjectStorage instances.
     */
    public function __construct() {
        $this->buttons = new Tx_Extbase_Persistence_ObjectStorage();
    }

    [...]

    /**
     * Setter for buttons
     *
     * @param Tx_Extbase_Persistence_ObjectStorage $buttons One or more Tx_LalAppfactory_Domain_Model_Button objects
     * @return void
     */
    public function setButtons(Tx_Extbase_Persistence_ObjectStorage $buttons) {
        $this->buttons = clone $buttons;
    }

    /**
     * Adds a button to this branch
     *
     * @param Tx_LalAppfactory_Domain_Model_Button $button
     * @return void
     */
    public function addButton(Tx_LalAppfactory_Domain_Model_Button $button) {
        $this->buttons->attach($button);
    }

    /**
     * Removes a button from this branch
     *
     * @param Tx_LalAppfactory_Domain_Model_Button $button
     * @return void
     */
    public function removeButton(Tx_LalAppfactory_Domain_Model_Button $button) {
        $this->buttons->detach($button);
    }

    /**
     * Remove all buttons from this branch
     *
     * @return void
     */
    public function removeAllButtons() {
        $this->buttons = new Tx_Extbase_Persistence_ObjectStorage();
    }

    /**
     * Getter for buttons
     *
     * @return Tx_Extbase_Persistence_ObjectStorage A storage holding Tx_LalAppfactory_Domain_Model_Button objects
     */
    public function getButtons() {
        return clone $this->buttons;
    }
}
?>

class Tx_LalAppfactory_Domain_Model_Button extends Tx_Extbase_DomainObject_AbstractEntity {

    [...]

}
?>

TCA:

$TCA['tx_lalappfactory_domain_model_app'] = array(

        [...]

        'buttons' => array(
            'exclude' => 1,
            'label'   => 'LLL:EXT:lal_appfactory/Resources/Private/Language/locallang_db.xml:tx_lalappfactory_domain_model_app.buttons',
            'config' => array(
                'type' => 'select',
                'size' => 10,
                'minitems' => 0,
                'maxitems' => 9999,
                'autoSizeMax' => 30,
                'multiple' => 0,
                'foreign_table' => 'tx_lalappfactory_domain_model_button',
                'MM' => 'tx_lalappfactory_app_button_mm',
                'wizards' => Array(
                     '_PADDING' => 1,
                     '_VERTICAL' => 1,
                     'edit' => Array(
                         'type' => 'popup',
                         'title' => 'Edit',
                         'script' => 'wizard_edit.php',
                         'icon' => 'edit2.gif',
                         'popup_onlyOpenIfSelected' => 1,
                         'JSopenParams' => 'height=350,width=580,status=0,menubar=0,scrollbars=1',
                     ),
                     'add' => Array(
                         'type' => 'script',
                         'title' => 'Create new',
                         'icon' => 'add.gif',
                         'params' => Array(
                             'table'=>'tx_lalappfactory_domain_model_button',
                             'pid' => '###CURRENT_PID###',
                             'setValue' => 'prepend'
                         ),
                         'script' => 'wizard_add.php',
                     ),
                 )
            )
        ),
    ),
);