Commented Unassigned: Invisible Update after import [306]

After a successful import of a custom package file the corresponding 'vendor' entry under the updates tree disappeared and the update is not displayed. When i try to import this package again the message "There's already an update with this Package ID. No import will be perform." is displayed.
Is there a way to cleanup this invisible package and restore the vendor group?
Comments: This setting is unchecked. The updates should not show up in the wsus console.