3076122 - Newly-added field via Data Source Extension is empty in the output XML file of an existing Sales Order

You're using the Data Source Extension feature to extend a Sales Order output with a certain field. This field is available in the output XML file of the existing Sales Order but the content for this field remains empty even though there are contents that should be populated.  

SAP S/4HANA Cloud

The Sales Order is outdated. After extending the sales order output with a certain field via Data Source Extension, you need to create the sales order again. 

Create the Sales Order again and then you will find this field and its content in the output XML file. 
If this field is on Sales Order Item level, you can also try by adding a new sales order item to the existing sales order.

SAP Help Portal | Create Sales Order

Sales Order, Output, Output Item, Data Source Extension, Custom Fields And Logic, XML File, Empty, Content , KBA , SD-SLS-GF-OC , S/4HANA: Output Control on Sales Documents , Problem