If a spider resets within a certain radius of where it initially spotted you, then it won't leash back - and so might end up on a different platform.
For example, you could aggro a spider next to the final branch leading to the top of the pit, web it on the branch, and then exit the map. The spider would reset, but would not leash back, since it was within still within a small radius of where it initially spotted you. As a result, it'd now path around on the branch.
Now, if you repeat that again, you might be able to lure it to the top.
A monster leashing back is simply a protection so someone else cannot train a monster on you - if the monster loses them as target, and it's too far away from where it initially entered combat, it'll run back, ignoring anything on its way there. Luring monsters to different platforms is rather tedious, and shouldn't really cause much of a problem - however if it *does* become an issue (ie. people mass-luring monsters onto a single platform), I'll do something about it.
