Are you sure that human task is completed properly? What I suspect is that when you reload the session and register work item handler for Human Task the handler is not connected to task server so it will not get notification about completed task. Try with invoking connect method on the WSHumanTaskHandler.
Best would be if you could provide maven project with process and test case, that would allow me to quickly review what's inside and confirm if that is a bug or not.
HTH