Nostalrius Begins - Quality wow vanilla realm (1.12)

about a week ago,when I'm herbing [Mountain Silversage] on this mountain,I notice this strange thing:
you can see a yellow point on your map (Mountain Silversage),but when you get a little closer,it's just....gone!so I thought someone take it,then I go away,but it is come back on your map!!!!even you can see it!!OK,I go to take it,just a little close,it's gone again!!it's really like you're walking between 2 worlds.

now see this:
on picture 1 you can see some fire evader on my left side,fine,let's move a few yards(take me about 3 seconds),now you can see picture 2,see?just like I'm in another world.
this thing only happen when you near this mountain,nowhere else in the whole game.anyone can tell me why this is happening?

TWO WORLDS COLIDE, WHO WILL WIN

Same thing happened to me at Fire Plume Ridge but also with mobs at the Tar Pits.
It's like you phase in/out whenever you are in a zone where elemental invasion mobs spawned.

Might be caused by the arrival of those mobs in the zone and/or the last patch applied. Think it's fixed since server restart though, isn't it ?

It's called Level of Detail (LoD)
https://en.wikipedia.org/wiki/Level_of_detail

This is an LoD bug.