Special Purchase Order No. Missing from Sales Order Line

We created a sales order (Drop Ship) and we created a Purchase Order by using the Order >>> Drop Shipment >>> Get Sales Order.

When we do that - the Purchase Order Number will populate the Special Purchase Order No. on the Sales Line.

We noticed that for two Sales Orders - it is missing the Special Purchase Order No. on the Sales Line. Is there anything that could cause this field not to be populated or if it got populated to disappear?