Last year I was traveling via YYC-YYZ-US desination with a checked bag - my connection was shortened from ~3 hrs to ~60 mins due to mechanical. I was SE at the time, so a concierge had met me at the connection and escorted me through customs to the gate. I had enough time to hustle through customs and security, but because my connection was below minimum connection time, I'd been offloaded, even though I could physically make the connection. I'd arrived at the gate while the flight was boarding, but had been offloaded and the gate agent wouldn't put me back on the flight because my bag wouldn't have time to make it and was being retagged for a later flight. Even the concierge couldn't (or wouldn't) do anything.

Unfortunately, AC seems quite eager sometimes to offload pax when their connection falls below MCT, even if it is a "doable" connection. If you only have carry-on, you have a better chance of getting back on the original flight, but once they've offloaded you with checked baggage, my experience is that they won't let you on, even if you make it to the gate during boarding.