Submitting a bug report as requested by Cian on the steam forum.
This happened on the Hell's Deep map, the final My Pet Dungeon. The problem is that creatures will sit around the outside of the prison and stave because they won't eat and seem to be caught in a pathfinding loop. I think what's happening is they are trying to eat the rats inside the prison. Beasts will remain stuck in this state unless a beastmaster shows up and drags them off to the arena. Deconstructing the prison may unstick the beast if they find a rat on the ground and eat it, but this doesn't always work. Filling in the room where the prison was will prevent further beasts from getting stuck, but if any beasts are in the room they will be trapped in solid dirt when it is filled in.
Attachments:
output_log.txt
hell's deep_20180802043103
hell's deep_20180802043103.meta
No one seems to be near the prison, can you give me more to go off