I and a bunch of people have been camping Gurfurlur for almost 3 weeks+. We've been watching the room and where it spawns the whole time. I can positively tell you its position never moved. Most of us did GM calls, and they actually did maintenance on Halvung. It must be bugged! I hope the maintenance opened its window to spawn next besieged. I will edit this post to update on what happens right after they did maintenance last night.
Edit: I just killed the NM. GMs did maintenance specifically on Halvung area, 8 pm last night. I got a /tell today, about 14 hours after maint. saying it had popped, and Trolls were lvl 0 Training. And they were still lvl 0 Training when we engaged and killed the NM. There had been no Troll Besieged for over 33 hours according to a up-to-date Besieged status site. So new tip: Gurfurlur can spawn at lvl 0 training after a maintenance.
Picture proof from killing today and Besieged site: