All speculation until OP answers the questions posed above: 1) one ticket all the way through? 2) who was it booked with.

Most likely culprit is UA for not updating. How would LX have a clue if the record isn't updated? All they know is pax didn't show for a segment so they cancelled him out.

Not that a pax should have to, but for future reference, anytime you are rebooked in IRROPS, make sure that the agent who rebooks you cancels out segments which won't be used anymore and that you reconfirm the onward itinerary which each onward operating carrier once you land.