I’ve done lots of winter camping, in tents. It takes a lot of experience. Forgot to put my boots in the bottom of my sleeping bag and had to dig through snow to find them, but only once.
It can get tricky, having a heater but not carbon monoxide. Most likely, that was their cause of death. That’s why three dead.
You need a constant inflow of outside air, no matter how cold it is.
Good observation. Probably what happened.