I just had a similar experience Saturday. Ticketed IAH-YYZ on UA then YYZ-onward on a UA-marketed AC-operated segment, with a 5 min schedule change I accepted weeks prior. OLCI, get both BPs, fly IAH-YYZ, hit the AC lounge in YYZ, no problems.
Go to board and AC wants my FIM... I don't have a FIM, there haven't been any IRROPS. AC says UA never sent the ticket over. Fortunately I'm well prepared and I've recently verified all the coupons are on the ticket and I have the ticket number ready for them to manually attach.