They definitely changed the visibility priority for S-rank monsters because of this issue a while back so that you can now see them from a mile away. I can't imagine that it would be too hard to do the same thing for A ranks; SE obviously wouldn't need to make them visible from so far away, but at least make it so that they always appear if you're with 30 yalms or so. There are only ever a maximum of 2 A-rank monsters in a zone at once, so this shouldn't put too much stress on folks' consoles / computers, either.
Even on a newer PC, there are times when the game will not load the A rank monster on my setup because there are so many people - I will have to be literally right up on it to see it in such instances. This always has the fun side effect of tons of people dying to monsters like Mudman on Tuesdays because they can't see what he's casting until it's too late. As an added bonus, if you get killed while in mid-air as you approach the monster, it teleports your body back to the nearest aetheryte, but there obviously won't be anyone there to rez you, and so you have to teleport back into the zone; if there are enough people to cause this issue in the first place, chances are that the monster will already be dead by the time that you make it back.