That would be unfortunate, if true.

I think they'd be well within their rights to not check bags through if they're on separate PNRs/separate tickets. AC has had a more generous policy in this aspect than some other airlines.

I would be surprised though if ticket stock mattered, though, if all flights were on one ticket. For example - lots of transborder flights through YYZ/YUL/YVR might be ticketed on UA ticket stock, with the first flight on AC, and I can't imagine AC wanting people to reclaim bags midway through.