When I create a Customizer panel like here:
add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
$meta_boxes[] = array(
'id' => 'general',
'title' => 'General',
'panel' => '', // THIS
'fields' => array(
the meta box unexpectedly appears in default post type (post) edit screen as well.
I needed to add 'post_types' => false although it wasn't documented here.