Not a bug: as you guessed, they aren't triggering a reduction because they're Impact abilities, not Ice abilities. Most "introduce a mechanic monsters" tend to not use the mechanic particularly well to give you a chance to adapt to the new thing. Alma do have Flash Freeze, which reduces the cooldown for Glacial Shell.
Thanks for the report regardless though, still good to know this is a bit confusing. I'll see if I can find some way to clarify it a bit.