This isn't really a bug, you are just doing it wrong :P
Temperatures are capped at -273.15C (absolute 0) to 9725.85C (10000K). When the temperatures go outside these range, they are fixed when the game is unpaused.
If you set the temperature to a really high number, you are actually hitting the maximum cap for an integer, which is 4 bytes so about 2 billion. After that it overflows and turns into a negative number. For me, it always sets it to -2 billion for the first frame, and when I unpause it gets capped to the proper 0K. I can't see why or how it would be -1K, but if it is it would only be for a frame anyway.