When pulling llama caravans and other animals via a lead across lower half slabs that have no block below them, all animals will attempt to take a route that avoids crossing the specific half slabs.  This happens every time when crossing half slabs, not matter the block type, if there is no block beneath the half slab.

If there is no alternate route for the animals to take, the one with the lead attached will be dragged across the half slabs while the other members of the caravan will not follow.  A way to recreate this is creating a bridge across a gap with lower half slabs and no blocks below them.