This is intentional, there are bugs if you try to push slimes against the level boundary but because of puzzlescript limitations it's awkward to detect if that's happening. So instead I prevent you ever getting a slime next to the level boundary.
Hopefully it's not too confusing! It's not really the cleanest solution but I hoped it wouldn't be very obvious.
Makes sense! I first noticed in A3, where it turned out that I could push the 1x1 slime south off the platform, but couldn't push it north.
I think the most confusing part is that it affects entering slimes, and the effect propagates. For example, if I define a level like
section Level A0 ...$. .s.s. ..... .s.s. ..... ..p.. _____
then it's possible to enter the right-hand column of slime (because it runs into a wall) but impossible to enter the left-hand column of slime (because it runs into an invisible boundary).
