Oda only said that CoO was his best haki.
Since he never even showed koka I am unsure what you expected from his CoO?
Its all good though. He can tank sword strikes from YC2 now without even looking.
Maybe just maybe the fact that Sanji almost always wears dark colored clothes in a black and white manga makes it a little bit hard to to tell whether he's using it or not as opposed Sanji somehow not being able to use an ability that almost every haki user in the new world can use

Fact that this is still a ting even now is baffling to me
Anyways on OT, Sanji's already shown an advanced application of CoO when he evaded Kat's jelly bean which was a feat Luffy was only able to accomplish just prior to learning FS.
Identical panels even up to the reactions but mofos would claim it's speed

He was also the only one that was able to sense that Robin wasn't on Elbaf which is a pretty insane given how big the island is irregardless of whether that type of CoO is characteristic to Sanji.
I don't think Sanji is ever gonna get the kinda panel time Oda dedicated to Luffy to learn FS coz he's simply not the main character. He will just show shit when he needs to (example being the panel above). Should be pretty obvious that Sanji is pretty skilled at CoO if you're actually paying attention and not being disingenuous. With regards to FS in particular I reckon he'd show it as some point or maybe even some other new OP CoO ability in the future when he faces an opponent that requires him to flex it.