Hello!

I would like to give it a try and give what I think. Kushina gets her immunity but she also loses it to other Kushina because of the Mystery Skill's effect "Suppress Immunity". It's just like Fire Main's Mystery Skill "Illusion - Mirroring Vision", it will not carry if you get attacked with something like "Suppress Immunity" before having the Fire Main's skill.

The passive itself is the one that makes Kushina Immune.