correct, they don't auto cancel any of your pre-made reservations under the old companion's name.

one reason is there may be a possibility you will be changing BACK to that companion before the flight date if it was booked quite a while ahead of time.

Their software does an automatic CP lookup report as a part of the final boarding reconciliation process, so although your ex- Companion would get a boarding pass and likely board the flight, WN would be calling their name before pushback to speak to them, the same way they do things for ticketed passengers holding a green must ride pass that try to separate it from their boarding pass and keep it--I've seen it done as have a few others on this forum have reported the same.