If a character casts a skill, and then receives frostbite whilst carrying out that attack, they then take damage from frostbite for that turn.
IE: Igorok has his Frostbite Taunt active. Torchy casts Fiery Fangs and receives Frostbite. Torchy then takes frostbite damage for having cast a skill whilst having frostbite.
I'm preeeetty sure it shouldn't be that way, right? Fortunately, it's a quick fix. Instead of checking if a character cast a skill and has frostbite, you cache whether or not they have frostbite before launching their attack, and then read from that cached value to determine if they should take frostbite damage!