Hard to say whether it's a bug or not given I do not know whether I am supposed to receive a message whether the emperor accepted or not.
I had challenged emperor Zaffasurf on chaos for a 2-3 month period, but nothing came out of it. I never received a message saying anything happened, or not. I have been told it is bugged because the emperor is located on Freedom and that prevents me from successfully being able to challenge him even though it gives me the message saying you challenge the ruler. I would like this fixed at some point.