Yes, this is correct behavior in SAP. Inventory and non-inventory items cannot be billed together in the same billing document. At the time of billing document creation, SAP will split the delivery into two billing documents - one for inventory items and another for non-inventory items.
This is done to maintain the separation between inventory and non-inventory transactions in the accounting. Inventory items involve stock/goods movement and valuation, whereas non-inventory items are direct expenses. Keeping them separate in billing helps maintain the integrity of inventory and non-inventory accounting entries.
So in summary:
- Delivery can contain both inventory and non-inventory items
- But at billing, SAP will automatically split and create separate billing