Activiti multi-instance sub process concurrent completion issue

Question

Multi-instance sub process is set isSequential="false", while sub processes are created in parallel without any issues, users are not able to concurrently complete (Rollback due to optimistic lock exception) the user task (defined inside the sub process). This is quite a common use case, else completing them in sequence take lot of time. Issue, remains even if multi-instance is designed on user task itself instead of enclosing them in sub process. The optimistic locking is caused due to multi-instance in built variables concurrent update(e.g. nrOfCompletedInstances and so on), not because of any custom variable updates