Update to my question. What I have noticed is that on Cmmn Case completion/terminate the Cmmn engine doesn't delete the Case (top level) execution record from 'act_ru_case_execution' table and the corresponding variables for this case/record from the 'act_ru_variable'. All the child executions(task, stage etc) and corresponding variables gets cleared.
This behavior is different from Bpmn process/engine. For Bpmn process all the process and variables data gets deleted on completion from the run time tables.
Is this by design or is it a bug? Does it have anything to do with History turned off?