Turkish Airlines Changed My Seat Despite It was Paid Seat (Flight TK2633)

Before the THY TK2633 flight from Malatya to Istanbul, my daughters' seat numbers were assigned as 9C and 9A during check-in. I had a different seat number. Since my daughters are young, I paid an extra fee to secure seat 9B for myself so that we could sit together. However, when I boarded the plane, another passenger claimed that 9B was their seat. I showed through the mobile app that 9B was indeed my seat. Both of us had 9B on our boarding passes. Later, with the help of a flight attendant, I found out that my seat number had been changed to 12B despite my payment. I do not understand how my seat number could be changed despite the payment and how my 8-year-old and 3-year-old daughters could be seated next to a person with a different surname. The images of my boarding passes are attached.