Custom taxonomy not shown by MEta Box

Hi.
I have created a custom taxonomy which I want to provided as a select option in a custom post type.
The taxonomy ist registered this way:

    $args = array(
      'labels'                     => $labels,
      'hierarchical'               => false,
      'public'                     => true,
      'show_ui'                    => true,
      'show_admin_column'          => false,
      'show_in_nav_menus'          => true,
      'show_tagcloud'              => false,
      'show_in_rest'               => true
    );
    register_taxonomy( 'event_format', array( 'event' ), $args );

I added the meta boy using this code:

        [
          'type'       => 'taxonomy_advanced',
          'id'         => $prefix . 'format',
          'name'       => esc_html__( 'Format', 'events' ),
          'taxonomy'   => 'event_format',
          'field_type' => 'radio_list'
        ],

Same code with taxonomy category is working:

        [
          'type'       => 'taxonomy_advanced',
          'id'         => $prefix . 'format2',
          'name'       => esc_html__( 'Format', 'events' ),
          'taxonomy'   => 'category',
          'field_type' => 'radio_list'
        ],

So, I assume there could be something wrong with the registration of the custom taxonomy. Maybe a flag is missing. What do I have to do to get the custom taxonomy visible in my taxonomy selector?