I think y0u are confusing heat loss with a device's power dissipation.
The power dissipation is determined by the power loss in the device and does not vary with ambient temperature.
The temperature of the device increases until the power dissipation to its surroundings equals the internal power dissipation of the device, as determined by the thermal resistance to ambient.
Spice does not determine that temperature rise.
You can determine the rise by measuring the device wattage (dissipation) in Spice, and then calculating the rise from the device's thermal resistance to ambient, which is determined by the device's package, how its mounted on a circuit board, and whether it has a heatsink.