Fire conversion to cold and using Tul's Stillness

Hey, so im using the new Talisman, Fury of the King and it converts 80% of physical damage to fire, and then im using the Goldcast Cuffs to convert 100% of that fire to cold.

Im messing around with support gems and tried to use Tul's Stillness and it does not seem to work. Tooltip damage does not change, i did try a low level fire ball spell to test the damage with and without the gem and its the same.

Any ideas why this is? and i chose bug reports because i do think this is a bug.

Just wanted to know. Thanks in advance!