No idea how they manage to do it, I've never seen it. But quite often I see my workers chilling out in my own prison. In most cases spirit workers for some reason. Especially if I have many workers, like the first level of new campaign where you get tons of free spirit workers via artifacts.
@fireeye
this bug is known internally, but we don't know how to replicate it. If you have any Idea how to replicate, please let us know.
It appears that you can get temporary spirit workers into your prison by directly dropping your spirit worker potion into the prison (however, once picked up, the temporary spirit workers can not be placed back into the prison).
How spirit workers created by artifacts get into the prison I can't say. My best guess would be that it's either a collision issue if many workers try to put corpses into the prison at the same time, or an issue caused by their blink ability.
Closing due to lack of customer response.