Are you skipping the task after completing it? or replacing the .complete with the .skip= You should skip a task when it is "Created", "Ready", "Reserved" or "In Progress", not when it is completed.
I have made a simple test for skip:
https://github.com/calcacuervo/JBPM5-Samples/blob/master/human-tasks/src/test/java/com/test/HumanTaskTest.java test skipHumanTask(), and works fine.
Regards,
Demian