Turkish Airlines Refund Issue for Los Angeles-Istanbul-Sofia Flight

I previously purchased flights from Turkish Airlines for the route Los Angeles - Istanbul - Sofia and Sofia - Istanbul - Los Angeles. Given that my plans often change due to work, I inquired in detail with customer service about whether I could cancel my return flights after completing my outbound flights and receive a refund. I received multiple confirmations on this matter, and the records are available with you. Later, I changed the dates of my flights due to changes in my travel details, again under the supervision of your customer service. They assured me that I could cancel the return flight after completing the outbound flight and receive a refund without any issues, but if I did not complete the outbound flight, I could not cancel the return. This is also recorded in your system.

Now, having completed my Los Angeles - Istanbul - Sofia flight, I called to confirm the information and possibly discuss a cancellation, only to be told by your customer service that the cancellation process for the ticket had expired after the flight and that only taxes could be refunded. Despite confirming the matter multiple times on different dates and acting under their supervision, I am now being told something entirely different, causing me significant inconvenience. This is an unfair practice unworthy of such a long-established institution. Additionally, I have been a loyal Turkish Airlines customer for many years. This treatment is unacceptable. I demand a full refund for my return tickets as promised. Otherwise, I will take this matter to the necessary legal authorities.