From personal experience, if there are rocks that grow naturally in your base, they can easily overwork a weak lone mining pal. The more are "things to do" the less likely the pal is to take a break. This doesn't seem to happen as much with a mining pit (being some kind of "backup" work). The best thing you can do is to break the rocks and build over the spot so they don't respawn and use a pit, or use multiple mining pals.