Okay this bug will bug the devs.
I did not read anything about it … maybe I did miss it.
I can not place floor or road on this area anymore:
The area will stay blank.
I do not know how this happened but before I encountered this bug I had a house on the same place and the floor did just vanish.
Maybe I did erase it with the rubber from the building tool, since I did try out some road placement and deleted part of this with the rubber a few times.
Here is the link to the save file:
Oh I am using some mods:
And I am using the latest steam unstable branch build