So, I built a base on top of the rock wall to the East. I first put down clay for a foundation along with walls, floors, and roof without a problem.

The first issue I noticed is that the floor is blended graphically into the clay instead of sitting on top, I would have expected it to appear above the clay without needing to use scaffolding.

The second, and bigger issue, is that any time I try to build an object (chest, lean-to, lamp, etc...) on top of the floor the blueprint is never shown despite it being in build mode. I have to cancel that build, go outside, and start the build process outside on the rock to get the blueprint to appear then go inside and place the object.

How to reproduce: go on top of a rock, place the foundation, place flooring on top of the foundation, try placing objects on that flooring.

I can probably record a video if necessary but have never done so before and would need some instructions for doing so.

I'm running into the exact same problem.  Thanks for posting the work around though.  I will try it later on.