Issue with Seat Selection on Turkish Airlines Flight

On May 16th, my wife and I purchased tickets from THY months in advance to visit our grandchild. Since it was going to be a long journey, we had to buy seats for each person and flight, costing us over €30 each. THY didn't use to charge for seat selection, but we accepted it as the market norm and purchased the seats. However, when we later tried to change our seats using the THY mobile app, we couldn't. We called THY customer service, and the last representative we spoke to suggested an unreasonable solution: if we canceled the seats, we would lose the seat fee we paid and would have to buy new ones. We couldn't buy new seats and had to stick with the old ones. This experience completely diminished our satisfaction with THY.