You have forgotten evaporative cooling. The larger the surface area of the hot container (bath) and / or the lower the humidity, the faster the hot bath will cool down! Another law of physics. Sorry.
Without constant renewal of heat, you are bound to cool off.
Kodak suggests starting a degree warm and finishing a degree low so that over the development time, you get an average "good" temperature. So, start and 101, and end at 99 in 3' 15" for C41.