I don't play Warchanter so can't be sure, but is there any chance they are actually making their save?
In which case the "white" VFX is still applied to denote the "even if they make the save, they take 20d20 Cold damage and are significantly slowed for 6 seconds" portion of the ability?
Testing in game suggests both effects trigger (slow effect and the frozen), the slow effect will always apply the white snowy icy look over you even if you make the save.
So I believe you are correct here in that the enemies OP have been facing have been making their saves against the Frozen effect; testing in A Sharn Welcome on Casual I was able to Freeze targets solid (so they can't move at all), besides the white frozen look slow effect.
Also of note, is that the effect is a cone and directional, meaning if you fail to target something first before using, there will be zero effect.
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The Damage and Slow portion is a Fortitude save, whilst the Frozen is a Reflex save. To be honest, given the lengthy cooldown of this ability and inability to Quicken as well, I would suggest grabbing a different Capstone if at all possible with your build OP.
J1NG