I don't know that is a bug or not, but this not work properly.
Try this:
Map<String, Object> parameters = new HashMap<String, Object>();
parameters.add(varName, varValue);
org.drools.runtime.process.ProcessInstance processInstance = ksession.createProcessInstance(processId, parameters);
ksession.startProcessInstance(processInstance.getId());
WorkflowProcessInstance wpi = (WorkflowProcessInstance) processInstance;
ts