I had reached around 1700 points in the AI Tier, and suddenly frostbite was active in an AI Tier Ranked battle. I thought it was suppose to be disabled in AI mode? I checked my festival task list to make sure I didn't accidently click on player battle, but player battle is still listed as 0 in my festival task list. I did a couple of more AI Ranked battles, but frostbite didn't come up again, even when my score was over 1700 points.

(I lost that battle since I didn't expect frostbite to be active, but I got so many tokens in my inventory and in the game and only lost 2 points so it isn't a big deal.)

I'm not 100% sure but I want to say it begins as you reach higher tiers of the AI to make the game more challenging.

Frostbite begins around tier 4, I believe.

In that case, I shouldn't have had frostbite at tier 2 / 3. It hasn't shown up after I got more than 1700 points, and I'm at tier 3 right now with around 1800 points.

The tier numbering system is a bit odd. It actually works like this...

Tier 1: 0-600
Tier 2: 601-1200
Tier 3: 1201-1800
Tier 4: 1801-2400
Tier 5: 2401-3000
Tier 6: 3001-3200+

So at 1800 points, you've /cleared/ tier 3 and are moving into tier 4.

Also, plus or minus about 100 of each threshold, it'll sometimes give you fights in the tier you're about to enter (or, sometimes, the one you just cleared). So it wouldn't be unusual to see some tier 4 fights (and frostbite) around 1700 points.