Seat allocated to standby passenger in my presence

I am disappointed by service received this morning at 6:00 am on 17 October 2022 where I was informed that I am already late for the flight and the official asked me my seat number and then called in a guy that was on standby to go through. This is highly unacceptable and I had to go back to book another flight for 13:15 because a standby passenger was allocated my seat. My ticket number was FLZJWJ. Lady informed me ACSA has their rules and they have their own when I asked why did I pass through the first security pass .