Okay, a bit more testing, and things are even weirder than I thought they were.
Basically, all of the non-damaging effects of Cone of Cold are being dumped on the last creature hit with it. Lets say you attack a group of 4 with Cone of Cold. Two of them succeed the save, two fail. All four take damage. The last target gets hit with two stuns, two times single turns of frigid, and two times three turns of frigid for a grand total of a stun, all stuns after the first wiffing from stun immunity applied the first stun, and a metric fuckton of frigid on one enemy in particular. It's wild, man.