Starting combat with Blizzard the mobs do not aggro.

Additional data. 

Flamestrike will aggro the mob, just not the channeled Blizzard.    Testing with a warlock, using rain of fire, which is also channeled, had the same issue as Blizzard where the mob takes damage, but then health resets.

Additional data points that might help narrow in on the root cause.  If you have a talent point in "Improved Blizzard", which adds the chill effect to your blizzard, then the mobs aggro when you start combat with Blizzard.

Testing on core M1, if you start combat with the unmodified Blizzard, the mobs do aggro properly.