REST API callback function is not called

Question

I have a very simple function which used to work perfectly fine. Somehow the callback function is not called anymore:

add_action('rest_api_init', 'create_api_posts_meta_field');

function create_api_posts_meta_field()
{
    register_rest_field(
        'comp-tourseite',
        'parentpagename',
        array(
            'get_callback' => 'get_parent_page_for_api',
            'schema' => null,
        )
    );
}

The function get_parent_page_for_api is existent and there were no changes made to the code at all. What could possibly be going wrong? Did something change since the last WordPress updates which I did miss?

I know that the function is not called since my logs inside the callback-function are not firing.