Nameless Seer not spawning [Bug reference number 463,654,858]

Hi

Like said in the title, Nameless Seer was supposed to spawn in my map but he just didnt. I had gone through every part of the map to see if there were any monsters left but none are remaining and he is nowhere to be seen. Maybe I am blind or something but the last time he spawned for me I could immediately see him on the map

Here is the image for proof:



Any help would be appreciated