After KeeperFX version 0.39a.
The Prison room is broken.
Enemy imps can also be dragged into the prison which I don't think is supposed to happen, and human creatures don't get converted to a skeleton after they die from starvation.
Then later on I ran into an unknown issue with my own creatures attacking the enemy's Dungeon Heart that was using my own imp as health bar 1 as the enemy's Dungeon Hearts health, but it was invincible, so the Dungeon Heart didn't get destroyed, neither did my imp in the middle of it, and all my creatures were just attacking my imp in the middle of the enemy's Dungeon Heart.
Not sure if it's also to do with the prison bug or not.