i'm not entirely sure, but I guess monsters fall into some kind of "sleep" state if the vis leaf they're in isn't rendered at the moment. this can be observed in Questionable Ethics when you enter the room with 2 alien grunts and filled with tripmines. If you go into the room above and hide in the left corner next to the window while the human grunts are breaching through the blast door, when they finish, there won't be any fight unless you take a peek at what's below through the window. Probably the visleaf the grunts are in gets hidden when you're in the corner, but I'm not sure about that.
I've had this problem in one of my maps too, where monsters in a monster box outside of the main map refused to react to scripted_sequences meant to teleport them in, unless I put an info_overview_point inside (or whatever that vhlt entity was that allows you render the whole map if you're in the same visleaf as that entity) and set it to reverse, so that the monster box is rendered no matter where you are in the map.
I doubt Valve have used hacks like that in their maps to warp monsters in, so there has to be an explanation. But for now, you should try using the aforementioned entity and reversing it by checking the corresponding spawnflag (don't forget that the entity is only present in the newer compilers), so that the room with the grunts is always seen. You also should get rid of this hack when an actual solution is found.
Tbh because of things like that I've mostly switched to Quake. Wish HL's ents were more reliable.