Did you make sure to leave 1 person on guard when this happened?
Yes. Especially on the latest occurrence mentioned, the one where I had my friend play just yesterday (so the memory is really fresh in my mind).
She got really lucky and had a decent roll the day before it happened so she was able to get enough wood and food for all 7 Campers, had 2 medication already prepared, and had someone on guard. In addition to that, they were all healthy and fed. Then the day ended, and we saw an immediate game over/all campers died screen.
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.