I do not know where to report bugs, however, this has been an issue for over a year it seems, as I had gotten a request from Garios to siege and conquer the city of Poros for 20k denars (which I did successfully). However, when I finished, I got a pop-up saying the quest has failed because I had not participated in the siege (which I did). I reloaded my save file over a dozen times to see if I did something wrong but it was always the same result. I went searching online and found a Reddit thread that exactly had the same problem as I did, which by the time I saw it today was already a year old. The entire siege and conquer quests seemed to be bugged/glitch, since it happens to any character who assigns the quest to you for any city.