It doesn't. How could it? Why should it?
Why overcomplicate things?
The vacuum bottle is merely an insulator. It inhibits energy passing through it.
Hot coffee? Lots of energy inside, wants to get out but can't get out because vacuum bottle. Therefore coffee stays warm.
Ice tea? Very little energy inside, wants to suck in energy from outside but can't because vacuum bottle. Therefore tea stays cold.
really? ok mr. thermos smart guy, then explain THIS:
you can blow on your hands to warm them in the winter, then go inside and blow on your soup to cool it? Hnnnn? So Explain that.....