Modular page not recognised (while another is)

Hi,

I’m creating a new website using Grav, which has two very different modular pages (each using a different template, etc). Both are organised with subfolders starting with underscore, and have content: items: '@self.modular'. However:

As you can see the /hoeje page works well and is recognised as modular. But the /kort page is not. In the admin UI (left), you can see that the subpages are not seen as modules, their IDs are not with _ and the template does not refer to modular/. But actual folders (right) have the underscores and are no different to the /hoeje page.

Does someone has an idea of what is going wrong? What does Grav uses to determine if a page is modular or not? I’m meant to finish this website tomorrow and I cannot make any progress while this page is not set-up correctly.

Thanks!

False alert: the reason was that these subpages had a slug in their front matter. Now that I removed these, it’s working.