The only known cause for this is using a body mod with a mismatched skeleton that's not compatible with it. When the bed is used, your character gets stripped of the prison clothing and your new equipment is added based on what you started as. It is this process that causes the issue when a skeleton mod that doesn't match the body is used. Something to do with bones the body mod is looking for that the skeleton doesn't have. Oddly enough, this tends to only happen on female characters. I have no idea why that is.