product_by_supplier: product_id cannot be visible opening a seller_ids item

After installing the module 'product_by_supplier',
If I try to open a 'seller_ids' item (located under the label 'Suppliers') from the product form view, I get the Product item (corresponding to 'product_id' field) as visible (and required).

This behavior is right only in case this form is open from the menu 'Product by Supplier' where I need it to link the related product. In the other case, the field many2one is unhelpful since already filled from the orm.