If I try to book on Google flights PTP to FDF on June 24th it finds the flights you talked about. Air Caraïbe. And then if I try on the same day from FDF to Yul it also finds the Air Canada flight but searching directly from PTP to Yul does not work. I don't recall that Google flights restricts itself to code share flights. So it is a little weird it doesn't find the connection on its own