Fire is Sanji's original genetics power, not the invisibility.
But why does the RS give an invisibility instead of a fire power like how the rest of his siblings with their respective power? The answer is simple, it was because Sanji was a failure in his childhood. None of his genetic modification come to fruition. That's why Judge designed his RS with an invisibility power so that even though he was a failure, he could still be useful.
But why does the RS give an invisibility instead of a fire power like how the rest of his siblings with their respective power? The answer is simple, it was because Sanji was a failure in his childhood. None of his genetic modification come to fruition. That's why Judge designed his RS with an invisibility power so that even though he was a failure, he could still be useful.
Like sanji's gen of germa obviously isn't the only generation that ever existed. There were likely previous generations of germa, previous stealth blacks.