I don't believe the energy consumption would be higher, but a plausible mechanism is this: When your neighbor heats their apartment only, their radiators need t

I don't believe the energy consumption would be higher, but a plausible mechanism is this: When your neighbor heats their apartment only, their radiators need to produce more power than if all apartments are heated. To generate more power, the radiators need hotter water. Producing hotter water lowers the efficiency of the heating system. So there is a tradeoff curve between higher heat loss (because more parts of the building are warm) and heating efficiency. There might be a point where it makes sense to heat more of the building, but I doubt that somehow.