OneNote Notebook seen as a folder in OneDrive Online

Hello!

I don't know what has happened but my OneNote (for Windows 10) Notebook in OneDrive has lost part of its magic and now it is seen as a folder by OneDrive Online (and not as a single file). See Sandro's Notebook below:

In Windows Explorer it appears as the standard link

It also works as expected on both OneNote Online and OneNote for Windows 10. Does anyone has any idea what has happened? And, if I may, how can I bring back its default behaviour?

Note: this is the customer version of OneDrive and, of course, no SharePoint here.

Thanks!