Simply tired of Mango

Flight 826 George to Johannesburg 8/9/19 Checked on line at 16h40 on 7/9/19 row 4 Only to arrive at the airport on 8/9/19 and be told someone paid for row 4 and we have been moved to row 17 Why does the functionality on the App permit check-in on restricted seats ??? Explain Mango ??? And to irritate the process the flight is delayed by 30 mins as at 1**** . In typical Mango style .... more likely to be delayed even later , again in Mango style