I've gone through the logs for this, and your return was rebooked onto a new flight number under the same booking reference. TW saw the matching reference and treated it as the flight already on your trip, so the new number never showed up as a change. It didn't quite trigger the signals we needed to recognise the renumber and update automatically.
The return has now been added to your trip and the underlying problem addressed, so that rebooked flights like this show up correctly from now on.
Thank you for reporting.