Septimius Severus from Santa Maria Capua Vetere

It has been reported (by ARCA) that a portrait of the emperor Septimius Severus that had been stolen from the Antiquarium of Santa Maria Capua Vetere in November 1985 has been seized at Christie's New York (June 2020) after being offered at auction by them on 28 October 2019, Faces of the Past: Ancient Sculpture from the Collection of Dr. Anton Pestalozzi.

According to the stated history, the head first passed to Jean-Luc Chalmin, London. Chalmin also supplied antiquities to the Stanford Place collection and to the Fleischman collection (apparently the fourth most important supplier). The Severus then passed to Galerie Arete in Zurich in 1993. (The same gallery appears to have handled ex-Becchina material.) It was then sold to Dr Pestalozzi.

How did the portrait appear at auction when it had been reported as stolen? Was the history of the piece checked? What was the due diligence process?