@Stefano Mandelli ,
Bookings Graph API does not have feature parity with the Bookings Web App yet, so multi-customer scenario won't work in Graph APIs.
Adding support for multi-customer scenario to Graph API is something that we have in our active backlog and we don't have any ETA that can be shared publicly for this yet. Please check https://www.microsoft.com/en-us/microsoft-365/roadmap?owM365RoadmapSearchInput=bookings&owM365RoadmapSearchButton=&filters=Bookings for the upcoming features for Bookings and we will keep this updated when Graph APIs will be updated as well.
Thanks,
Babu (MSFT)