Greetings, Raim!
Thank you for taking the time to post your concerns about the spawning of the Prince of Pestilence NM. It seems that there have been previous reports from other players who experienced similar concerns with the NM, and the matter is currently being inspected by our development team. I recommend keeping an eye on http://lodestone.finalfantasyxiv.com/pl/index.html for any further developments. Thanks again for your report!