Non player craft falls through surface of planet when approaching with player craft

I had landed a craft on Luna, and later, i landed a rover roughly 33km away. As part of my goal with the rover, i targeted and approached the lander. Everything was doing fine, with me saving every five kilometres, but when i came within 1km of the rover, a text message came up saying 'Targeting Luna 1". It started to display distance in metres, but then the target started to descend underground. I tried saving and exiting, but then all trace of the targeted lander was erased. The screenshot is where the lander should have been.