Fire Penetration does not apply to Fairgraves' Curse Spectral Fire debuff

I have 52% fire penetration on my character from the passive tree.
I also have the Fire Penetration III support attached to Phantasmal Arrow.
I fought Jamanra (30% fire resistance) with a +40% elemental resistance map mod, for 70% total fire resistance.
I stored a 4.6 million damage Spectral Fire debuff on Jamanra. When it triggered, it dealt 1.3 million damage, or 30% of the stored damage, indicating that the full 70% fire resistance took effect to reduce the damage dealt. I have a recording of this if needed.

This means that neither my characters' implicit fire penetration from passives, nor from the support gem attached to the phantasmal arrow skill, applied to the spectral fire debuff hit.