I decided to try again just now (only minutes ago), to see if I'd get more lucky, but had the same thing happen again. I wasn't paying attention to the day, but it was after day 16, and we were past where we moved to the second floor.
There were 6 campers alive. I had been extremely lucky on rolls just before this point so I was very confident with my resources. I had someone on guard, over 20 wood (was trying to prepare for the upcoming 20 wood check), over 10 food, 3 medicine, and no one was even exhausted that night. Then the day ended, and I got another instant "All Campers Died" screen.