Possible Herald of Ice + Cold to Fire bug?

When linking Cold to Fire support to a Herald of Ice skill gem, only the AOE damage portion is being modified -- the small flat damage bonus to spells and attacks is not affected. Is this a bug or intended behavior? I would expect each of the cold damage portions to convert separately.

If your main skill is a fire based spell or attack, then being able to link Cold to Fire with Herald of Ice would take advantage of any fire damage % boosts you may have. This is especially true for skills that are not likely to do enough damage to shatter enemies and the AOE portion of HoI would go unused. Incinerate is a good example.

I'm also think about this, thats why im refresing this question.

It's intended. The only damage Herald of Ice itself deals is the area damage resulting from shattering enemies. So naturally that's what cold to fire linked to it applies to. The damage it grants to attacks and other spells is not dealt by the herald of ice skill, but the actual skills that the damage is added to. If you want that added damage to be affected by cold to fire, you need to link it to the skill you use to deal damage.