Adding a building that eliminated unhappiness (Unyielding Order, at least) stop revolts, but just having a building (in this case, Demonic Citizens, which also eliminates unhealthiness; the Fallow trait is what eliminates the need for food) in a city does not prevent revolts. There can be many causes of revolts other than the vanilla ways of conquest and culture, mostly through random events. In this case though, I suspect it might have to to do with building Beasts of Agares. These always cause revolts, and kill 4 population.