From my understanding total implied variance has to be a monotonic function of time for there to be no calendar arbitrage. Stumbled upon quotes for this Monday with apparent arb (NKE Dec expiry vs Jan), not sure if this is correct, but I have checked this across open hours and it looks persistent throughout the day.
Is this even possible? Am I missing something?
Vols are derived from mid of top of the book quotes. There are no divs before the expiry.