Welp - finally had a run in with AC's OLCI. Booked with UA as a codeshare flight.
Have been having weird problems with my booking ever since I had a schedule change that had to be resolved through UA a few months ago (my outbound leg completely vanished, and I was left only with an inbound leg). I've since been unable to view it under "My Bookings", unable to select seats, etc. on AC's website. Any activity on AC's website with the AC PNR yielded an error, "please contact us to request a refund", or something. Prior to the schedule change, this was never an issue.
Contacted AC, and they assured me everything was good to go with the booking. OLCI opens, and now saying that my flight is not available for check-in. Frustrating, as I was planning a fairly late arrival at the airport tomorrow due to work commitments... doesn't leave much room for whatever surprises I end up encountering at the airport.