It’s a hassle with all airlines and travel agents (and company policies) to some extent.

My crude understanding is that it stems from who has ‘control’ of the ticket but there may be lots of other factors like being on a non public fare, a company policy that doesn’t allow it, the wrong type of leaves on the line etc.

if ticketing had been designed in the last twenty years or so it would be trivial but though bits of paper have long gone the entire business process seems to be designed and if anything we seem to spend time thinking up even more complicated processes (eg dual inventory fares)