To get the discount belonging to your subscription you need to check in and out with the OV-chipcard on which the subscription is loaded. You can not and are not allowed to buy an e-ticket with discount in the app for that. The option that was there before was a Travel Together ticket which is only allowed to use if someone travels with a subscription holder. The subscription holder always needs to use the subscription that is on the OV-chipcard and check in and out with that card.
What should I do for my next trips? I don’t want to experience the same problem.
Just check in at the start of your trip and out after the trip with your ov-chipcard. Then you will have an automatic discount when you are eligible for it. There is no need to buy supplemental tickets in the app. You will have the discount automatically.
You can request a refund of the overpaid amount at https://www.ns.nl/klantenservice/geld-terug/geld-terug-bij-vergeten-abonnement.html
Next trip: always check in and out with your OV chip card. The system recognizes the products on the OV chip card and will charge the subscription rate if applicable..