here's the problem why the slaves aren't mining anything sometimes, when they all go to the node(s) that spawn HERE
and their AI just shuts down, they all get stuck there like the pathing for them doesn't work, I would just change to have no nodes spawn there and put a new node(s) on a wall to make up for it...far away from that area, or just widen that tunnel or something.
to fix it ingame you have to tell them to follow you again and bring them out of that area marked in red (the squiggly is the node spawn that bugs them, and im too lazy to turn off player names for a screenshot)
I haven't had them disappear yet or anything.
Also you might not be able to set a merc to stay or guard a private mine, they wouldn't let me tell them to do so at a mine, but they do so at a tent/safehouse.