In the Invoke Workflow activity, on editing value for In argument, click the + icon to open in advance editor, here the existing value is not coming in the editor.
I experienced something similar with the ‘Invoke Powershell’ activity here.
The best course of action I’ve found is to save my current work, close the project completely, open the project, make a needed edit and save. If the arguments need edited again, I suggest following the procedure again.
This is a know issue/limitation. We will address it in one of our future releases.