Turkish Airlines Ticket Change System Issue

I booked tickets for 5 different passengers through the Turkish Airlines app for a round trip from ESB to NEW, using a total of 4 PNR numbers. I opted for the Economy Flexible (the most expensive economy class option available on Turkish Airlines). When I attempted to change the return date, the app presented me with two options: Economy Semi-Flexible (L-Y Class) and Economy Flexible (M-Y Class). I paid the additional fee and confirmed the ticket rules for the M-Y Class. However, the system erroneously changed my ticket to Economy Promotion (V-Y Class), which I never selected. Why would I make a change to a ticket class I never saw in the system? Additionally, I only intended to change the return flight, but the system incorrectly downgraded the class of my outbound flight as well. I was asked about changes for both the outbound and return flights, but I proceeded with only the return flight change. The system error is now being reflected as my fault by Turkish Airlines. I initially paid 48,119.92₺ for these tickets (Economy Flexible), but now my tickets have been downgraded to the same class as tickets costing 32,522.92₺ on the day of purchase. I've been flying with Turkish Airlines for 20 years, and this is the first time I've seen the airline blame customers for their system's issues. Additionally, I was charged 40.95₺ for the change fee. My total refund amount is 15,517₺, and the fare difference is shown as 15,476.05₺. This clearly indicates that I only changed the return flight, yet the system downgraded all my flights. As a consumer, I request that only my return ticket be updated according to the classes I saw in the system.