Got reply from Robert Mohr and Jon Yusam from Qlik that the request should actually have the Json object like this
that means the selections and connectionId are not properties with config object. I have changed my code accordingly and the selections now is working like an additional filter and NPrinting is applying the same filters logic, that mean, if more than one value list is applied to the same field, only the common values among the list of values would apply. I haven't tried the scenario with the override flag in a filter though. Would try out later and see if the selections is considered to be a filter at the end of the chain of filters or a the front.