Found in DWachs GUI-Patch, but surely has nothing to do with it.
You shouldn't be able to create what I attached. Just use the tools as numbered in the picture: First you build the tunnel, then you lower the full tile. This allows the low-slope-tool to be used on the tile, which flips the slope direction - already breaking the connection. I lowered the tile again so it's flat. This is repeatable, moving the tunnel down a layer each iteration.