Which is why I acknowledged that there may be other issues at the table. Such things should be discussed out of game time. The fact that it possibly worked before could be part of the issue.I don’t think simply having the monsters follow the ranger up the rope is the part most people are taking issue with. On its own I think that’s perfectly reasonable. But if it was done to “teach the player a lesson,” rather than because the DM thought that was genuinely how the orcs would react, that’s a problem. Regardless, it seems like the DM and the player had misaligned expectations, and that should be addressed, independently of whether or not either party’s tactics were sound.
I'm just saying that it doesn't really have anything to do with the enemy having detailed knowledge of how the spell works. Assuming they know magic exists, they have a pretty good guess what happened and proceeded to take a logical response. I've had players who got upset because of things sort-of-similar to this, some people seem to expect the enemies will have the same level of intelligence (or lack therein) as they have in video games. I think it's a good idea to remind people that NPCs and monsters can be reasonably competent now and then. If they get upset when the enemy counters this kind of thing it's not my problem.