It was so long since I played D2 LoD that I couldn't remember... but until a couple of months ago I played D2R for a while and one thing that I really noticed is that the merc bug isn't there.
Yes it can get stuck in the corner of a room, unable to find a path to the player, but it's always trying to get to you, it always teleports to you sooner or later and it never goes into that immobile state that can happen in Median.
There's a small possibility that they fixed it between d2 and D2R but there's no mention of it in patch notes anywhere.
So this seems to be a Median thing, I hope it can be fixed one day because it makes it painful to play most characters without tele (which seems to reset the merc). Using tp/wp (which also seems to reset it) is not really viable.