Using a spiral borer in Bethesda Susa also forces a tunnel of caves to connect abandoned wards, etc

Reproduce: Use a spiral borer in the bottom right but one corner of Bethesda Susa surface
Expected: The borer bores a set of stairs which are often not connected to the abandoned wards, ruined wards, etc.
Observed: Map generation appears to force a connection between those stairs and the main area of the wards at each level