Flowable - sending signal for legacy in-flight processes fail

Question

We are on flowable 6.5 and have in-flight legacy processes which were created in Activiti 5.15 and continue to work in backward compatible flowable 5 engine.

Problem - we are sending signal in legacy in-flight processes and it fails in org.flowable.engine.impl.agenda.TriggerExecutionOperation with a NPE given ActivityBehavior activityBehavior = (ActivityBehavior) ((FlowNode) currentFlowElement).getBehavior() returns NULL from below code.

Ideally FlowNode behaviors are set by bpmn parsers which deploy the process definition in memory. Though for flowable 5 compatible process definitions the parsers set ActivitiImpl and not FlowNode. So when the signal is invoked , it gets in TriggerExecutionOperation and tries to find current behavior in FlowNode and cannot find it, given it was not set by bpmn parse handler for legacy processes.

Same flow works great if I am sending signal with a new process definition instantiated in flowable 6.5 engine.

Any ideas why this is happening ?

Code that fails in TriggerExecutionOperation

    public void run() {
        FlowElement currentFlowElement = getCurrentFlowElement(execution);
        if (currentFlowElement instanceof FlowNode) {

            ActivityBehavior activityBehavior = (ActivityBehavior) ((FlowNode) 
 currentFlowElement).getBehavior();
....
}

Thanks Binish