I don't know why it does it or if it has to be a specific circumstance, but this guy playing a frost amp BG kept getting +3 on his champs every time he equipped Cloak of Ice. They all had Frost basic attack but idk if that's relevant to the damage gain. Haven't tried to reproduce it either.
Thanks for the report. Codewise, it does give +3 DMG if the unit has Attack: Frost. I will look at it and make a ruling.