When you click/open the work item, it gets completed. That's why it is in the outbox. Completed work items cannot be forwarded (basically nothing can be done for them anymore). Most likely your step/task has been built in a wrong way. I have no idea what your step is supposed to do, but in 99% cases, when you have a dialog step in a workflow, there is e.g. some button that need to be pressed in order to complete the work item - for example "Approve" or something like that (depending on what kind of workflow process you are dealing with). And if you don't press that button, the work item will stay in your inbox. Steps shouldn't be completed just by clicking/opening the work item.
You need to tell more details about your step. What is it doing (what happens when you click the work item)? Or perhaps you can share the code of the method that gets executed in the task?
Regards,
Karri