Can't get simple taxons to work

Hello everybody,
I built a simple taxonomy tree with three levels and added a prdouct.


This the product page in the frontend:

Then I go up one level through the breadcrumb, the left menù shows the main taxon of the product but the list of the products in that category is empty:

I thought the test product was shown on this list but is showed only in the category above:

May someone help me understand what I’m doing wrong?

Thanks in advance.

Best regards.

Marco Bruni