Yeah, that had been my analysis, too. Pretty much by definition, all the bodies in a system with no other heat source will reach ambient air temperature given enough time. The experimental data from the Canadian study made me realize that there are two ambient temperatures for an abandoned house: the air temperature and the ground temperature, and the ground temperature will act as a heat source if its warmer than the air temperature.
It really is something we need to revisit, but I don’t code the temperature math so I’m not likely to fix the bug.
Also, interior air temperature is only going to stabilize near ground temperature if the house’s insulation is intact. As soon as a window gets broken and you get convective cooling of the interior air, it’s going to dominate the heat transfer over the conductive heating from the ground.