Basically you cannot provide a logical reasoning or explanation which would explain why Oda still draws the eyebrowflip and why Sanji has it when he tanks S-shark?
My explanation isn't headcanon, Sanji was already feeling weird.
The exoskeleton process was slow and developing, so it's a logical conclusion that in those panels his trasformstion didn't completely ending, but it was close to it since he started to show the exoskeleton rigeneration, after the eyebrow flip like a few pages after it was completed.