I was in YUL last week, arriving at 9pm and they were making announcements for all AC pax with next-day connections to pick up their bags as there was no transfer service at that time. If it was a shorter layover in the middle of the day, the luggage would be checked through.

The next morning you'll re-check your bags, clear security and then clear US customs in YUL and land at the domestic gate in MCO.