Unlike the Length of Stay/Ultimate Park Hopper tickets of the past that were tied into your reservation, had to match the number of nights you stayed and expired at midnight of your check out day, MYW tickets have nothing to do with the number of nights you stay. You don't even have to purchase a Disney package with tickets to match the length of your stay. Many folks stay for 5, 6, 7 days during the free dining and only purchase single day tickets with the package.
If a guest purchased 5 day MYW tickets for a 6 day/5 night stay and went to a park on arrival day, then again on each subsequent day, there would be no admissions left on check out day (arrival day used day 1 from the ticket; 2nd day used day 2, third day used day 3, fourth day used day 4, fifth day used day 5). So when you go to enter a park on day 6, there are no admissions remaining. You would have needed to purchase a 6 day MYW ticket, not a 5 day to enter on that last day.
Luckily, the ticket could still be upgraded for only a few dollars to add the 6th day, even after you have checked out of your resort.