As noted in several comments, non-breacher and non-sapper raiders will target and attack cobblestone walls even when there is an open path to other targets, which breaks killboxes and other defense structures relying on predictable pathing.

The fix is simple. Go into your the mod folder and look for Buildings_Structure.xml. It should be located in something like Mods/2023513450/1.4/Defs/ThingDefs_Buildings/

Look for <defName>VFEM_CobblestoneWall</defName>. Under this look for the corresponding <building> tag. Add <isInert>true</isInert> under the building tag.

For an example, you can just scroll down the same file to <defName>VFEM_LowRockWall</defName>. This one has the proper isInert tag. Other walls from the default game XML files also have this tag. For some reason it was missed for the cobblestone walls.