While sparring, most spells behave as expected: you cast them, it hits the enemy.
Hypothermia (and, at a guess, the other new spells as of the 2019 priest update) have a weird quirk however: if the opponent back off as it casts, the spell errors with "Vynora would never accept your spell on X" (Vynora was fine with it when they were in my face).
This behaviour is inconsistent with other similar spells (ex: Shard of Ice) and a detriment to sparring's purpose of emulating PvP.
This could be fixed by importing the line
if (!performer.isDuelOrSpar(target)) {
which is present in Shard of Ice and other similar spells, and missing in Hypothermia and Inferno.