Yea it's a pain because you don't know where the lines are but it does work. I've always seen every S or A I've been to and they only disappear if they get pulled away, the only way to load them again is if you run after them. The real problem is if you get there and you can't see it from the start, because in my drawing if you run right it wont load,back wont load and left wont load so if you don't know it's location very well you have about a 75% chance of going the wrong way. In that case you kinda just have to guess but when you walk into the right grid it's pretty evident when the mob is suddenly in your face. Also you can be standing almost exactly on top of the boss because even if you're 1 pixel outside that grid the game might not load it. Here's another crappy paint diagram to show how i assume the load priority works.



Inside each of those grid spaces the first thing checked is if there's odin or something similar and if there is it loads it, but if you have too many people in your grid then it wont check any past that cause you're already capped.