
It looks like this will have a satisfactory ending, but a wild ride. Don't know if it will be helpful to anyone else, but at least you'll get a laugh!
We booked Jan/Feb round trip on American via Chase Travel Portal last November. Changed the flights in early December to come back a couple days earlier, from a different city. In the leadup to the flight, we received the usual emails from American about international travel, received seat upgrades, and were invited to check in online (which we couldn't do, but that has been normal for covid era international travel).
Arrived at airport 0430, went up to the counter (luckily, DW gets us priority, as the lines were otherwise long), and were told that we had no ticket! Talked to the agent, saying that we had upgraded seat assignments given to us the day before, and that American had been emailing us about this trip. Apparently, that is a different computer system, and the one that counts didn't have us listed.
Called Chase travel (the one bad thing about booking through a portal rather than directly with airlines is having to jump through extra hoops when things go sideways). Luckily, got a quick answer--but not the answer we wanted. "My system shows you are booked, and since the flight is soon, I can't mess with it. In any event, this is the airline's fault."

Back to the counter (at this point, we are really really glad about DW's priority status, as the main line was out the door). There are tickets remaining for Nashville-Miami-Medellin, but yikes! $1000 for each ticket--so the two combined are double what the original round trip cost. Bite the bullet and buy, only to discover that the ticketing counter software isn't integrated well with TSA precheck list, and AA's own frequent flier program. So he has to run everything 3 times to get us accurate ticketing/boarding documents....
Rush through airport, get to gate 5 minutes before boarding; a jewel of a gate agent managed to get us into the seats that we thought we had been upgraded to, and we made it to Medellin as scheduled.

Submitted complaint/inquiry to Chase travel that very day (Jan 20). On Jan. 23, they emailed saying they had accidentally cancelled the outgoing flight when making the return change and said they'd make it right. Per their request, I immediately submitted documentation for reimbursement--but as of today, hadn't heard anything else, until I called and prodded a bit....
Glad we had the ability to bite the bullet on this, but would be a vacation breaker for many--and there was no reasonable way for us to know the issue until we showed up at the airport. (A little silver lining--I put the "new" tickets on the credit card that we were trying to run up for SWA companion pass, and it enabled us to ring the bell a month earlier!)