Sheep keep falling through the earth

[Yellow.Legs]

We built a pen and a trough for our Ovis and set them all to wander. Within a few minutes they keep falling through the earth. We’ve tried this several times, restarting our server etc. and it continues to happen, again within a few minutes set on wander. Not sure if it’s the specific plot of land we built the pen on (it’s slightly hilly)?