The openWorkItem API will not return the page-activity details as part of pageFlowDetail when the pageflow is defined in a different project. The external pageflow is not available to the external client (SoapUI or custom apps) as the DAA of the external pageflow is not wired in the application that calls the pageflow. The reason the workspace or openspace that can open the external pageflow as part of openWorkItem is it is wired internally with an additional API getBusinessServiceDetailsByModule call.
Resolution
To get the details of the external pageflow, use the getBusinessServiceDetailsByModule API from WorkPresentationService that provides the page-activity details of the external pageflow.