I had a similar experience first time I flew, bloke said to me "oi, that's my seat" ballax I said, OK, you fly the fkin thing then.
I bought mine from ticket office last Saturday and my ticket has my seat number, row , gangway etc. On it.
So if it was the ponty that was open and not the east stand.? I also got my ticket in advance but it didn't bother me to sit unreserved . If I'd have been asked to move I would have done so but with the east stand 2 thirds empty why make an issue. Got mine in advance cos it was a quid and not a fiver.
Because my seat is in an ideal location for me, ie halfway line, It is a popular area as previous unreserved seating games have proved. If your not in early they’re taken. So I bought my ticket early to reserve my seat, therefore I have a right to sit in my seat and will do.