Hi Long,
Thank you. It works on a custom field, fine. But I want also change the post_content field that is loaded. The code doesn't work for that field. I have tried to make an extra filter like rwmb_normalize_{$field_id}_field but it doesn't work.
add_filter( 'rwmb_normalize_post_content_field', function( $field ) {
if( !is_admin() ) $field['type'] = 'textarea';
return $field;
});