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.
The same bug, or a similar one, is above ground as well. Strangely, the direction can not be flipped back.
Maybe one should also hide the directional tools for underground to stop giving player these ideas ;)
Honestly, I think it would be best if the landscape tools would not work for ways at all, and instead there were dedicated "way up" and "way down" tools. I suppose it works the way it does for historical reasons, but it's not intuitive at all.
The problem with dedicated tools is, that these need new pakset versions. This is the limiting factor.
No longer allowed in r9048