I think what's happening is that you have two enemy ships spawned, one directly above the other. The Thrusters get force placed when the ship is built, but they still have a build restriction of 70 tiles under them. For the top ship, this sometimes makes the Thrusters ghosts, or not spawn at all. For the bottom ship, it can "destroy" entities and platform foundations. This is a known limitation. You can check the ChangeLog for all the known issues. I'm testing a "fake" thruster that gets substituted for the real thing, which shouldn't have these placement limitations. I should have all that ready for v0.1.2. Thanks! Keep the bug reports coming.