Since a long time ago, sadly in this game, not in a galaxy far away, upward or downward flattened tiles, that you build palisades on, are not unblocked once you remove the palisades.
Before the last patches, you could unblock them by building above the tiles in GM mode, and removing it again. This "functionality" to deal with this bug is gone.
Remove the bug, or at least bring that "funtionality" in again, or better give GMs a command that can unblock the tile they are standing on.
This is with running time ruining much ob build space, and makes many places look ugly or be unusable.
BIG Problem