The placement location of the rovers in the upper slanted cave above the 2 green 1 purple Power Slug Cave in the SE bottom corner of the Oil Crater Zone Cords 127557/ 249292.
I tried to recreated the event when you place rovers in the cave they tend to move and have collisions with each other. If you go to lower slug cave and notice the rovers protruding through the cave wall triggers them all to fall through the floor. I was unable to get this to happen second time.
The rovers already have an unstable proximity with each other combined with the fact I had the Fog of War disabled and was able to look up and notice the Rovers on the cave wall. The fog normally is near white out here so most likely this area not a concern and the upper cave is not really designed for items placed there unless a foundation is locked in place first .