This is new feature in 0.2.0 floor can get destroyed by asteroids (and make your platform smaller)
This is not working as intended right now and will be fixed asap (already in progress for 0.2.1)
right now destroyed floor is grass which you can still build on and functions as platform. (this is going to be fixed as well)
In 0.2.1 I will also add option to disable this destruction
It appears that currently after warping the cement/etc tiles stay on the ground and are not mineable, and can't be tagged with the deconstruction planner. [tested with no other mods except transport drones, trying to see if the road network would work]