Short answer
HubSpot's KB: product sync matches on SKU and only syncs products with SKUs by default. Unmatched invoice line items map to a Custom Line Item product in QuickBooks. Two-way invoice updates only work for invoices created in HubSpot. HubSpot invoices don't support a Service Date for revenue recognition.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Add SKUs in QuickBooks before connecting to avoid duplicate products.
3. Related
QuickBooks invoices and field mappings.