I have a Forms process with a Submission and then Approval User Task.  The Approval task is assigned to the last submitter (/_Submitter). This worked OK.

After the Submission User Task, I added a simple Workflow Service that creates a Token for Year (to be used in a folder name).  It returns the Year Token to the forms Year field variable using 'Set Business Process Variables'.  As soon as I added the Workflow Task I lost the value of (/_Submitter) in the next step; Approval.  The process was suspended.

I have had to include a workflow token and form field for submitter in the form.  The workflow returns the submitter as well as the Year.  The Approval task is now assigned to my submitter variable and not (/_submitter).  Problem solved.