Removing ground level floors

egingell_1​  In your video, what I saw was that you were deleting the floor in the open area after the stairs were in place, not before placing them.  When I tried that, it deleted the entire floor in the room including the stair landing and area on both sides of the fence.  I didn't get the ground staying like you showed.  This issue hasn't been reported by anyone else nor have I seen any other reports with anything similar.  So, that means the issue is unique to your save or game.  So, a couple of things you can try.

If you haven't already done it, try a repair.

And if you are using mods and CC, particularly any mods that affect or used exclusively in build mode like T.O.O.L, try building in a vanilla save without your mods and CC to see if it still happens.  Don't just turn them off, move the mods folder out to test.  I did my test build in my main save with mods and CC active, but I use no mods that affect build and very little build/buy CC.  Also, make sure that testingcheats and MOO are off when you try.  

Hope this helps.