SunExpress Assigned Me a Middle Seat Despite Free Window Seat Selection During Check-In

I just made a flight reservation from Cologne to Ankara with SunExpress, and when I checked in, I chose a window seat at no extra cost. Surprisingly, the automated system decided to give me a middle seat instead, despite the fact that there were plenty of empty seats on the plane! Something seems off here, and I've come across numerous complaints about similar problems. I believe this will be my final flight with SunExpress. From now on, I will opt for a different airline.