First question: How are you posting the designs? Do you use GDBM or just the Post Design button in ArcMap? I think either way it ends up calling the Post Design task normally unless you have a very custom GDBM setup.
Also, can you check the Post Design task in the Px Admin Tool to see if it has the Verify Design subtask listed (it normally does)?
When you say WR polygon status you mean WORKFLOWSTATUS right?
The Verify Design subtask has a step at the end that "nullifies" designer fields.
One of these steps is to set the WR and DN workflow status to None.
It also sets the features in the design WorkFunction to None.
It transitions the Workrequest Px Node in WFM to Complete by executing the "Complete Work Request" task on the WR Px node OR whatever task name you have set in the Px Config "WMSTaskCompleteWorkRequest" value if that is set to something.
Remember that WR (and DN) nodes in WFM have Px States but features in ArcMap have WorkflowStatus with a domain. You can configure your Px States and Transitions and Transition Tasks to match your workflow but it's a bit tougher to change the WorkFlowStatus on the features in Designer.
The set values for WorkflowStatus in ArcFM Designer is:
mmd8wfsBuilt mmd8wfsCancelled mmd8wfsClosed mmd8wfsInConstruction mmd8wfsInDesign mmd8wfsNone
Correct me if I'm wrong but I think you would have to build a custom subtask that sets the WR polygon's WorkflowStatus. You would also need to add "Complete" to the WorkflowStatus domain and give it a new code (number).
Then you would configure this custom subtask to run after Verify Design subtask in the list for the Post Design task.
Thank you for your detailed answer.