It could happen because the task complete listener are not registered for some reason:
https://github.com/droolsjbpm/jbpm/blob/5.4.x/jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/process/workitem/wsht/AsyncGenericHTWorkItemHandler.java#L120
Maybe you can debug to see if TaskCompletedHandler(https://github.com/droolsjbpm/jbpm/blob/5.4.x/jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/process/workitem/wsht/AsyncGenericHTWorkItemHandler.java#L216) is being called..
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