Originally Posted by Often1

As a starting point, seat assignments are not guaranteed. That is made explicit in the COC. Passengers are moved for all manner of reasons, including to acommodate parent-child and caregiver situations.

Linking PNR's via PNR notes is possibly effective, but presumes that a human being takes the time to read the PNR notes during the limited time which exists to handle an aircraft substitution. For other than urgent situations, it is asking more than seems reasonable to preserve pairings when the passengers chose not to do so in their booking. And even then, there simply is no way to make everyone happy.

What is most telling is that OP chose not to politely ask the other passenger themselves, but sought the CSD's intervention.

There are several good discussions of this recently and the inherent pressure which this puts on passengers, perhaps not on experienced regular travelers unless the request is made discretely, out of the presence of the benefiting passenger and only after the uniformed crew member makes it abundantly clear that he is acting solely as an intermediary and that there is neither an obligation nor pressure to acquiesce. (This as oppose to a crew member order).

The reaction here was over the top and I suppose that the CSD intervened because calming the situation is important to getting the aircraft off, but this was a zero-sum game in which the uniformed crew intervention thumb on the scale was used to disadvantage someone else.