It is basically like the header says, but this did happen when there was a hero tagged Beast Master inside the prison and hero tagged beasts outside the prison.
For clarifications sake, was the Hero Beast Master placed into the prison in the map editor and hero beasts placed nearby or did the Beastmaster end up in prison during the course of play and he just grabbed passing beasts?
Gave this some serious testing today inside and outside of the map provided. Unfortunately couldn't replicate so it's possible that it's already fixed internally. I'm going to mark this as complete next patch so give it a try then but if you suffer from it next patch let us know.
Actually, this kind of behavior seems to happen in the Arena as well. What happens in this case is that it put itself and other random units inside of the Arena. In my cases it was 2-4 beasts, the Beast Master, and a worker. This also happened with hero tagged units.
So, my thought is that the hero Beast Master rallying ability is bugged.
That's certainly an odd one. Can you explain further the situation? IE what map, how far in, etc