At war with Drengin after capturing a planet from the Fundamentalists?

A number of planets from all races broke away to form the Fundamentalists. I captured one of the planets that used to belong to the Drengin. All of a sudden, I went from having warm relations with the Drengin to being at war. Does the AI realize that it is no longer their planet? Is this the expected behavior (i.e. once a planet belongs to a race, they should be the one to get it back after rebellion, or risk war)? I thought that once a planet rebels, it is free for the taking.