Hey I thought this was the case as well until a recent game as the Bannor in which I lost Trinity to the Infernal, everything corrupted to Hell terrain, and then I took Trinity BACK and oh I dunno maybe 100 turns later the Hell terrain suddenly disappeared. Can anyone else confirm that this is how it works? If that's how it's supposed to work, I like it. Corrupted land slowly heals itself when back under the care of a Good civilization.