This can actually work both ways, I truly believe that other than the NY area, most AC res agents are clueless about co-terminal situtations. I would be very surprised is you were not hassled at FLL if you checked in with a MIA-YUL segment assuming that FLL has AC check-in staff and not UA staff. The flight searches for YVR-YYC also pull up YXX-YYC which the computer recognizes as a co-terminal but the YVR agents do not. Arrrghh!
Good luck getting any sort of response from Customer 'Solutions' in Calgary, I have stopped sending in letters as of last year due to their poor response rate.