They get stuck inside the blocks. Oftentimes they flicker in their attempt to pick up their feet and walk farther. I have managed to destroy the blocks so they can walk another step, but they just get stuck in the next one.
I've done a few adjustments to nail this error down. My guess is that they can't get up the half-block step I had, but I can't confirm that yet. I've replaced the half-step with a stair-shaped block, and no one has gotten stuck on the plaza top yet, but I can't confirm anyone has wandered by to try. I tried to herd them towards the plaza by shooting the gun to get them to run, but I guess I'm not much of a cowboy as I can't get them to run in the direction I want.
Another hint seeps that these cows walked through a full block set into the ground, but these *don't* get stuck. It was only they when they were on top of my plaza.
I've built a little test plaza in a place they like to congregate and will watch and see what I can figure out. More soon.