Maciej,
In my case
$processInstance : WorkflowProcessInstance()
$request : Request($id:id) from $processInstance.getVariable("request")
not (exists (Issue(requestId == $id)))
is not a rule but the flow condition in a jbpm process.
So does that condition return "true" for the process instance "A" (requestId==1) if there are not any issues like Issue(requestId==1) in the session but there is another process instance "B" (requestId==2) in the same session and there are some issues like Issue(requestId==2) in the session as well?
Regards,
Dmitry Erkin