I've got a half dozen or so mining bots, and they have been mining down all day, but now they seem to have stopped digging down into the area of core fragments. If I dig down manually, they will fill in the platforms, but they won't make additional horozontal tunnels. Is there a level at which they stop working to avoid hitting the planet's core, or might there be something causing their AI to glitch?

I have a screenshot, but i rarely encounter bugs worthy of pictures, so I dont know how to upload it...