This is even more perplexing. Booked on a 014 ticket, on UA 6623 operated by AC 8658?

Is it possible Expedia created two PNRs for the booking, with a 014 on the outbound and 016 on the return? I have definitely seen that before with OTAs where it's cheaper to book two one-ways, so you end up with one Expedia reference, and one airline reference for each direction. Except in this case, you might have ended up with four airline references given the mishmash of marketing and operating airlines.

I've also had AC say "you're not booked on AC 123 today" because it's not on the PNR loaded on their screen, even after repeatedly stating "it's a separate booking and I can provide the reference".

So if you provided them with the outbound reference, then they may not have done the "look up PNR by flight number and name".

But this is obviously highly speculative. I just find it extremely unlikely you'd end up with a 014 ticket with a UA-marketed AC-operated segment.