I still remember what happened at YVR last year on March 1. I was flying HKG-YVR-(overnight < 24 hours)-YYZ. When I checked in at YVR, the check-in agent insisted that I give her a new certificate and there's no such thing as the 24 hours rule. She said she was going to downgrade me [she was in the process of doing it] if I don't hand one over. Since that was the beginning of the benefit year, I only had 8 and I needed them all for travels in March back then. However, I reluctantly handed one over and told her she's wrong.

I wrote to the SE desk when I got home and they told me I was in fact right and sent me a bunch of SE certs in return. Just make sure the agent who check you in at your originating city put a note in your file about this.