A recent data point.

AC304, I ended up the first place loser. Too tired from an early flight from Sydney, I just boarded a bit grumpy

Seated in PY. Had a nap for about 1 hour. The plane did not move. FA came over with a new BP crossing out my PY seat and a J seat handwritten in.

Slept in J for another hour and then everyone was kicked off the flight due to some engine issues.

Ended up rebooking myself to a flight via YYZ. When I boarded the new flight (was given J outright), AC304 was still awaiting its engine to be restarted.