The same thing happened to my wife and I in 2021 for a St Lucia flight, it was originally club world and I booked and paid for the good non-step over window and aisle, then it was switched a week before to CS and we were separated. I was able to get better seats and then 2 or 3 days before it was switched back to CW and we were split again, not ideal for a 9 hour flight. Customer services didn't help at all and there was a lot of shuffling on the plane with several unhappy couples and families. On our return customer services were not sympathetic and initially refused my request for the seating refund as "all the seats are the same"

This all could have been avoided with some common sense and manual intervention but "computer did it" is too easy answer.