My wife and I do not travel very much but we experienced a similar problem with Air New Zealand. We travelled on their "Pacific Premium" class and specifically asked for seats in a 2-seat row as my wife is disabled. We were promised that this would be no problem. When we checked in we found ourselves in the inside 2 seats of a 3-seat row and "our" seats became occupied by a very young, able, couple. No explanation was offered.
Travelling on 4 different aircraft on a return trip to Costa Rica with a US airline, we were allocated separate seats in different parts of the aircraft on all sectors!!! Luckily we were with a group so a bit of jiggery-pokery unabled most of us to sit with spouses/partners.
I really don't understand how some airlines function.