In your response I can see an an empty inline feed. This can be achieved by two ways
Option 1: Do not implement method /IWBEP/IF_MGW_APPL_SRV_RUNTIME~GET_EXPANDED_ENTITY( ). Id you have correctly implemented a read for 'ProductionOperationConfirmations' and a query for 'GoodsMovements', framework will call the methods and will return you the data.
Option 2: If you have explicitly implemented IWBEP/IF_MGW_APPL_SRV_RUNTIME~GET_EXPANDED_ENTITY( ) then you need to write custom logic o populate the data in nested structure. The nested structure will be having all the properties from entity 'ProductionOperationConfirmations' with an extra field called 'GoodsMovements'(navigation property name), the type of field GoodsMovements would be an internal table of 'GoodsMovements'.
Next you have to pass the NavigationPropertyName to parameter 'ET_EXPANDED_TECH_CLAUSES' to tell the framework that you have done the expand on your own and no framework attention needed.
I would say debug the method and check if you are sending the correct data in correct format .