When going over half blocks like a pathway or bridge where the half blocks are the only option,  it appears that the llama caravan pathing does not recognize the slabs as a viable option. If at all possible, the caravan will attempt to route around the llama on the lead (which can be dragged across the blocks).  If the caravan llamas get on the the slabs, they are stuck and need to be pushed onto a taller block, from which they will try to rejoin the caravan, if it still exists.

Discovered this issue in a bukkit 1.13.2 server and tested and successfully replicated the issue in the latest snapshot (18w50a)