Posts not searchable

Hi,

I've created a CPT and Meta Box. One of the metabox fields is a articlenumber(text-field) and I want it to list before the title and make it searchable.

The thing is: as soon as I add

'admin_columns' => array(
  'position' => 'before title',
  'searchable' => true,
)

I can search on the meta-key but not on the default title anymore. The search also lists
Search results for ""
without the searched word. As soon as a remove the admin_columns array it works as WP should out of the box.

Any ideas why this might happen?