Heat rises. That’s a basic fact of physics, nothing will change that. It’s the reason that the second floor of a house tends to get hotter than the ground floor. It can be a problem in both summer and winter, trying to cool down the house or avoid the upper floor from overheating.
However, an effective HVAC system for a home, particularly a newer home with good insulation, can strike the right balance and keep the second floor from getting too hot and create a good, even distribution of comfort through the house.
You, however, aren’t currently enjoying this situation. At least not if you’re reading this intently. Your house has previously not had a serious problem in winter with a boiling second floor. But now it’s become such a sauna where going outside in the cold seems like a better option. Why is this happening?