Builders are not deleting stockpiles for some reason.
Initially they left 1/4 of section with one plank on it and now I cannot delete this one section. If I create more stockpile around this 1/4 tile it also did not permit me to delete part of newly created stockpile.
The worst part that this is level where I need to build a huge monument and this is only one free space to do it and I now have understoyable stockpile in the middle