Merc not following in map if died previously

Title pretty much self explanatory. If merc dies in map and you res it, it won't show in the instance anymore.

Yeh that is design of game.
If the Merc dies, then it cannot join you again in the same instance.

You can make a new instance, (hold down CTRL on waypoint or the entrance to the area - select new) and they will re-appear again.

Didn't think it was intended actually, did ggg explicitly said that it is indeed intended?

When they die they return to town.
This is the reason they cannot join you again, on the same instance.

Also it is incentive,
to upgrade your merc items to stop it dying,
and also to protect it from dying in being careful on what you fight and how