Overlaying lower level constructs shouldn't damage spaces.

[Anonymous]

If, using PN, a construct containing walls is overlaid into another
construct containing spaces, the walls from the lower level become
boundaries for the spaces in current level construct despite the walls in
the lower level being below the cut plane.

In general, if a construct is overlaid into another construct, spaces should
not look for boundaries in those constructs. If that wish interferes with
built-in features, spaces should seek boundaries in their own level as
defined by the cut plane or should seek them within specified z limits.