Orphan Segment?
 

Booked on AC, ORD-YYZ-PEK.
First leg was on a UA flight, and I'm crediting the flight to my MileagePlus acct, so the flight showed up in my "Manage Reservations" section on the website.
The UA segment got rescheduled to much later in the day, and AC rebooked me on an AC flight for the original time.
However, the rescheduled UA segment still shows up in Manage Reservations, and I can change my seat and everything.

It's been a couple weeks since the schedule change, will this be auto-cancelled? Should I call and tell them this is still in the system? The segment doesn't show up on AC's website, so I assume there would be no consequences for my itinerary for missing that segment on UA.

Call United to have this fixed so that you will have a peace of mind before you travel.

If AC owns the PNR, I am not sure if UA will touch it. I would say ask AC to check it and see if they need to do anything.