Settings API - sanitize_callback is not called and it leads to an incorrect behavior

I am in the process of creating some options / the options page for my plugin. In the context of this I have only added two checkboxes so far to store boolean values. The creation, display on the page and saving works fine. Regarding future options, I wanted to test the validation of the options. For this I have created the function sanitize_options, which now simply returns the parameter it receives as a test.

public function sanitize_options( $data ) {
        return $data;
}

I passed the function name to the function register_settings as parameter.

register_setting(
            'faqdesk_general_options', //Group Name
            'faqdesk_general_options', //Name of the option
            array(
                'type'              = 'array',
                'sanitize_callback' = 'sanitize_options',
            )
);

Of course, this doesn't make much sense or serve any purpose. But as I said, I just wanted to try it out. But now I had to notice that this function is not called at all. In addition, the option value is filled with NULL when I save the changes on the page. If I take the value sanitize_callback out of the argument array completely, everything works as desired, so error-free.

What is the reason for this, where is the error? Am I making a stupid thinking error? Please help me out and excuse me if I am simply stupid.

Following the complete class:

?php

class Faqdesk_Settings {

    /**
     * Single instance of the class
     *
     * @var null
     * @since 1.0.0
     */
    protected static $_instance = null;

    /**
     * Singleton
     *
     * @since 1.0.0
     * @static
     * @return self Main instance
     */
    public static function instance() {
        if ( is_null( self::$_instance ) ) {
            self::$_instance = new self();
        }

        return self::$_instance;
    }

    /**
     * Add admin menu.
     *
     * @return void
     */
    public function settings_admin_menu() {

        add_submenu_page(
            'faqdesk_main_menu', //slug name for the parent menu (menu_slug is set add_admin_menu() function in the class Faqdesk_Admin)
            __( 'FAQDesk Settings', 'faqdesk' ), //page title shown in the title tag
            __( 'Settings', 'faqdesk' ), //name of the submenu displayed on the Dashboard
            'manage_options', //minimum capability required to view the submenu
            'faqdesk_settings', //slug name of the submenu
            array($this,'render_settings_page_content') //function to be called to display the page content
        );
    }


    /**
     * Renders a page with a tab navigation to display the settings.
     */
    public function render_settings_page_content( $active_tab = '' ) {
        ?

        div class=wrap

            h2?php _e( 'FAQDesk Settings', 'faqdesk' ); ?/h2
            ?php settings_errors(); ?

            ?php if( isset( $_GET[ 'tab' ] ) ) {
                $active_tab = $_GET[ 'tab' ];
            } else if( $active_tab == 'social_options' ) {
                $active_tab = 'social_options';
            } else if( $active_tab == 'input_examples' ) {
                $active_tab = 'input_examples';
            } else {
                $active_tab = 'general_options';
            } // end if/else ?

            h2 class=nav-tab-wrapper
                a href=?page=faqdesk_settingstab=general_options class=nav-tab ?php echo $active_tab == 'general_options' ? 'nav-tab-active' : ''; ??php _e( 'General', 'faqdesk' ); ?/a
            /h2

            form method=post action=options.php
                ?php

                settings_fields( 'faqdesk_general_options' ); //settings group name - call references the whitelisted option which had declared with register_setting()
                do_settings_sections( 'faqdesk_settings' ); //slug name of the page whos settings section should be outputed, therefore it outputs all the sections and form fields which has defined in initialize_general_options()
                submit_button(__( 'Save general settings', 'faqdesk' ), '');

                ?
            /form

        /div!-- /.wrap --
    ?php
    }

    /**
     * This function provides a simple description for the General Options page.
     *
     * It's called from the 'initialize_general_options' function by being passed as a parameter
     * in the add_settings_section function.
     */
    public function general_options_callback() {
        $options = get_option('faqdesk_general_options');
        var_dump($options);
        echo 'p' . __( 'Adjust the settings, which concern basic details and functions of the plugin.', 'faqdesk' ) . '/p';
    } // end general_options_callback

    /**
     * Initializes the general options page by registering the Sections,
     * Fields, and Settings.
     *
     * This function is registered with the 'admin_init' hook.
     */
    public function initialize_general_options() {

        // Register setting: The group name can be anything actually, but I think its just simpler to name it the same as the option that will get stored in the database.
        register_setting(
            'faqdesk_general_options', //Group Name
            'faqdesk_general_options', //Name of the option
            array(
                'type'              = 'array',
                'sanitize_callback' = 'sanitize_options',
            )
        );

        // Add section, which defines how the settings will be visually grouped 
        add_settings_section(
            'general_settings_section',                     // also HTML ID tag for the section
            __( 'General Options', 'faqdesk' ),             // Title that will show within an H2 tag
            array( $this, 'general_options_callback'),      // Callback that will echo some explanations about that section
            'faqdesk_settings'                              // slug name of the page which to show the section 
        );

        // Add settings fields, describes how to add the form input
        // SETTING: Cleanup on deactivation?
        add_settings_field(
            'delete_data_on_deactivation',                                  // also HTML ID tag
            __( 'Cleanup on deactivation', 'faqdesk' ),                     // Formatted title of the field, which is displayed as the label for the field on output
            array( $this, 'toggle_delete_data_on_deactivation_callback'),   // Callback function that will echo the form field
            'faqdesk_settings',                                             // The page on which this option will be displayed
            'general_settings_section',                                     // The name of the section to which this field belongs
            array(                                                          // The array of arguments to pass to the callback. In this case, just a description.
                __( 'Activate this setting to delete all data, when the plugin is deactivated.', 'faqdesk' ) 
            )
        );

        // SETTING: Cleanup on deletion?
        add_settings_field(
            'delete_data_on_deletion',                                      // also HTML ID tag
            __( 'Cleanup on deletion', 'faqdesk' ),                         // Formatted title of the field, which is displayed as the label for the field on output
            array( $this, 'toggle_delete_data_on_deletion_callback'),       // Callback function that will echo the form field
            'faqdesk_settings',                                             // The page on which this option will be displayed
            'general_settings_section',                                     // The name of the section to which this field belongs
            array(                                                          // The array of arguments to pass to the callback. In this case, just a description.
                __( 'Activate this setting to delete all data, when the plugin is deleted.', 'faqdesk' ) 
            )
        );


    } // end wppb-demo_initialize_theme_options


    /**
     * This function renders the interface elements for toggling the visibility of the header element.
     *
     * It accepts an array or arguments and expects the first element in the array to be the description
     * to be displayed next to the checkbox.
     */
    public function toggle_delete_data_on_deactivation_callback($args) {

        // First, we read the options collection
        $options = get_option('faqdesk_general_options');

        // Next, we update the name attribute to access this element's ID in the context of the display options array
        // We also access the show_header element of the options collection in the call to the checked() helper function
        $html = 'input type=checkbox id=delete_data_on_deactivation name=faqdesk_general_options[delete_data_on_deactivation] value=1 ' . checked( 1, isset( $options['delete_data_on_deactivation'] ) ? $options['delete_data_on_deactivation'] : 0, false ) . '/';

        // Here, we'll take the first argument of the array and add it to a label next to the checkbox
        $html .= 'label for=delete_data_on_deactivationnbsp;'  . $args[0] . '/label';

        echo $html;

    } 

    public function toggle_delete_data_on_deletion_callback($args) {

        // First, we read the options collection
        $options = get_option('faqdesk_general_options');
        $delete_on_deactivation = isset( $options['delete_data_on_deactivation'] ) ? $options['delete_data_on_deactivation'] : 0;
        $attribute = '';
        if($delete_on_deactivation == 1) {
            $attribute = ' disabled=disabled faqdesk-tooltip=' . __( 'Not so useful to set this when already at deactivation all plugin data is deleted...', 'faqdesk' ) . ' ';
        }

        // Next, we update the name attribute to access this element's ID in the context of the display options array
        // We also access the show_header element of the options collection in the call to the checked() helper function
        $html = 'input type=checkbox' . $attribute . 'id=delete_data_on_deletion name=faqdesk_general_options[delete_data_on_deletion] value=1 ' . checked( 1, isset( $options['delete_data_on_deletion'] ) ? $options['delete_data_on_deletion'] : 0, false ) . '/';

        // Here, we'll take the first argument of the array and add it to a label next to the checkbox
        $html .= 'label for=delete_data_on_deletionnbsp;'  . $args[0] . '/label';

        echo $html;

    } 

    public function sanitize_options( $data ) {
        return $data;
    }

}