[plone.restapi] @navigation -> items empty for migrated content items, visible after editing them

Env: Plone 6.0.0b3 with a fresh Plone site with content imported through collective.exportimport.

The items list of the @navigation endpoint is empty (in most cases) for fresh migration content.
The related content is public, the requesting user has admin rights. When we manually edit one of the subitems that is missing and saving it, the item is available in the items list. I am not getting the clue what could cause this behavior...any idea?

A closer analysis reveals that the problem goes away when an effective date is set manually or programmatically.