The ancient Seleucia was founded along the Tigris river around the year 305 BC, and abandoned in 165 AD, due to the river mutating its course (Wikipedia).
However in 780 AD the Catholicos (=Patriarch) of Seleucia was the most important Christian authority, overseeing over a quarter of the world's Christians . This fact alone suggests that the city was far from an abandoned site, and rather thriving.
Some sources refer to Middle Ages Seleucia as Seleucia-Ctesiphon. So let's assume that the Catholicos was based in Ctesiphon; the destruction of Ctesiphon was perpetrated by the invading Arabs, who later founded Baghdad. However Again there is an inconsistency, as the Arabs burned Ctesiphon (or Seleucia-Ctesiphon) in the VII century, and founded Baghdad one hundred years later. Where were the Catholicoi(?) based meanwhile?
All this is in disagreement with the most common view that Seleucia was abandoned because of Roman attacks and replaced by Ctesiphon in the II century. The neighboring Ctesiphon, was in turn abandoned in the VII century after the Arab invasion, while Baghdad was founded in the VIII century.
Everything appears like a giant naming mismatch.
: The Lost History of Christianity, P. Jenkins, Harper Collins