I would be interested to know who says that a product with a Rabbanut hechsher on it is makpid on yoshon. As far as I have always know, only Badatz and possibly the mehadrin hechsherim like Shearis and Rubin, are makpid to give their hechsher only on a yoshon product.
Products that are yoshon in chutz la’aretz have a notation about that status near the hechsher. There is a Rav Herman, who was recently niftar, who created a whole guide on products and when they would become chodosh, investigating thousands of products to find out their status and putting out a guide for those interested parties, free of charge.
A Rabbanut of Israel stamp on a product produced in chutz la’aretz has no more assurance of its chodosh/yoshon status than the original hechsher. A prime example would be granola bars from Nature Valley sold here in EY. You would have to refer to the chodosh guide to find out when those products began using chodosh products in order to know whether they are still yoshon.
If anybody has conflicting information on what I have written, please advise.