In hindsight, I think I know why I wasn't able to book it on AC. I was booking YQB-CPH on one of the cheap sub $2200 J fares, I couldn't build it on ITA Matrix and despite having given all the flights to the AC agent it was only available on Exepdia and a bunch of other more dubious OTAs. I figured it out when I saw the reservation on the AC app: its actually a YQB-CDG fare with a 22 hour layover in CPH and a 3 hour turnaround to the return flight back. Lesson learned, again.