I know what's happening here. In the original games, trigger areas are always locked to a 4x4 grid. Almost all original levels account for this by also aligning object positions to the same 4x4 grid, but this level does not do this with the exit object, so the game moves the trigger area slightly up and left so it's aligned to the grid. This raises the trigger area enough for the lemmings to exit without digging.
SuperLemmini, on the other hand, does not have this restriction, so the trigger area stays buried. The next version (which should be out within the next few months) will have a fixed version of the level, with the exit raised slightly.