boss outside the map

i encounter the immured fury spawned outside the map. He didnt jump down so i decided just to kill him when he was up there. I dont care about that loot but i just think its weird that he can spawn up there. I was just wondering what if he dropped hinekora or mirror lol. Should i just stand and eventually wait longer till he jumps down or if he is bugged on this map. heres the video https://youtu.be/JnNQQ0mqUXI