Copycat Slabs seem to just utterly break Villager pathfinding, particularly if there's a wall that's one and a half blocks high using a Copycat Slab on top. I have the paths of my villagers lined with these one and a half block tall walls, and villagers will sometimes move around them in stupid ways, or get stuck just jumping next to them thinking they can climb up it.
The amount of times I've found a bunch of villagers jumping in the same spot trying to jump over a 1+1/2 tall wall that they can't get over, it's just breaking the workings of my village in a really weird way.
I suspect it's because villagers don't recognise copycat slabs as actual slabs because when I replace them with regular stone brick slabs they work fine.