Who was the first Eastern Roman emperor to visit Rome (in the 7th century) after the fall of the Western Roman Empire in 476?

It was Emperor Constans II.

Constans II reigned from 641 to 668, and in 663 he made an important trip to Rome. This was a rare and remarkable event, as Rome had been outside the control of the Eastern Empire for almost two centuries. Constans II's visit to Rome was part of his efforts to strengthen his influence over the Italian peninsula, which was still under threat from both Lombard invaders and the growing power of the papacy.

In 663 Constans visited Rome for twelve days-the first emperor since the fall of the Western Roman Empire in 476 and, along with John V Palaiologos, one of only two Eastern Roman emperors since the division of the Roman empire in 395 to set foot in Rome-and was received with great honor by Pope Vitalian (657-672). Although on friendly terms with Vitalian, he stripped buildings (including the Pantheon) of their ornaments and bronze to be carried back to Constantinople, and in 666 declared the Pope to have no jurisdiction over the Archbishop of Ravenna, since that city was the seat of the Exarch, his immediate representative.

https://en.wikipedia.org/wiki/Constans_II

https://www.doaks.org/resources/online-exhibits/gods-regents-on-earth-a-thousand-years-of-byzantine-imperial-seals/rulers-of-byzantium/constans-ii-and-constantine-iv-654201359