Yes this threw me for a couple of turns - 'invulnerability' is good in some ways but ARR also encourages you with the kiting strat from what I have seen in quite a few battles and it's annoying when you have a mob down 50% on the edge of its leash and you run over the barrier and it runs back while you are still trying to hit it. Would it be useful if there was some sort of 'you are about to pull the mob out of its instance' warning?
Yeah, but then people would complain about the game catering... lol, j/k. Yeah, it would be nice if they made a translucent barrier line, mathematically the information exists... However, that's a significant increase in render objects, so, I imagine that would be a balancing act for them.
What I have done with some fights where it mattered was tag the mob to find it's leash range, then have the mages stand just on the inside. That way we don't have to worry about it.