Get a custom fields value from a settings page

I can remember I did run into this before, but I do not know how I solved it.
There must be something obvious I am missing. I used Meta Box AIO for code generation and did look at https://docs.metabox.io/extensions/mb-settings-page/#getting-field-value, but I can't see anything wrong.

I did a bare-bones theme for testing:

functions.php

<?php

//all code is generated from Meta Box AIO

add_filter('rwmb_meta_boxes', 'custom_fields');

function custom_fields($meta_boxes)
{
    $prefix = '';

    $meta_boxes[] = [
        'title' => __('custom_field_title', 'your-text-domain'),
        'id' => 'custom_field_title',
        'settings_pages' => ['settings_page_titel'],
        'fields' => [
            [
                'name' => __('Text', 'your-text-domain'),
                'id' => $prefix.'custom_field',
                'type' => 'text',
            ],
        ],
    ];

    return $meta_boxes;
}

add_filter('mb_settings_pages', 'custom_settingspage');

function custom_settingspage($settings_pages)
{
    $settings_pages[] = [
        'menu_title' => __('settings_page_titel', 'your-text-domain'),
        'option_name' => 'settings_page',
        'position' => 25,
        'style' => 'no-boxes',
        'columns' => 1,
        'icon_url' => 'dashicons-admin-generic',
    ];

    return $settings_pages;
}

index.php

<?php

get_header();

$value = rwmb_meta('settings_page', ['object_type' => 'setting'], 'custom_field');
        var_dump($value);

get_footer();

I would expect the output to be:
.../wp-content/themes/test/index.php:12:string 'lorem'

instead I get:
.../wp-content/themes/test/index.php:12:boolean false