Hello!
Pretty much as the main question, on my server today could appears Shrolg, I went to the cave and there is a guy killing the bog raiders, I was outside waiting but he stay there for like 30 min, he actually kill the bog raiders, and stay on the safe spot and then run again to kill the bog raiders. He also used the coal basin, for no reason.
I'm not sure, but is possible that of do this, he could "block" the spawn of the boss?