Why would the scheduled ship date not being populated on a drop ship order

This is a new problem and it only occurs sporadically. We have many drop ship orders that we load and update thru the APIs. In 95% of the orders, the scheduled ship date is automatically populated and the goods can be received and the order closes.

Now, we are having a few orders where the scheduled ship date is not populated and we can not receive against the PO. The request date type is "SHIP" for these ship-tos but that has not changed. Should it be Arrival?

THe orders have their request date changed, for example an order booked on Jul 10, with a request date of Oct 1, may on Jul 20 have it updated to Oct 12. But this occurs for all the orders.