Originally Posted by Twickenham

I'm not quite sure what was going on in your search, but I just ran the same search (YYZ-IST, IST-CDG, CDG-YYZ), and I had no problems getting options for all 3. And just to further emphasize the point, the random date I selected had no availability on the YYZ-IST TK flight, nor on any non-stop CDG-YYZ, so my final test itinerary ended up being YYZ-FRA-IST-CDG-YUL-YHZ-YYZ, which really pushes the limits of allowable mileage.

As another data point, I have YQB-YUL-IST-BCN booked for next summer, and have frequently seen YUL-IST-BRU offered by Aeroplan.ca, so I think either it's a glitch or your arrival/departure times on the connection didn't line up.