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Currently, when you go to invoices, all invoices appear in a page setting of 50. If you change the filter to display only unpaid and click save as default it doesn't save it. After you view an invoice and return to the invoice scree list you have to again change the filter to display the unpaid invoices. This has been reported previously and not fixed.
Thank you for your feedback, we try to keep the ideas portal for just feature/enhancement requests and this appears to be a bug in the system. I have redirected it over to the proper team so they can take a look into what may be going on. Thank you for your patience!