Cap

If "Cap" is enabled, it will generate a dynamic mesh to "fill the hole".

The Cap is a Dynamic Mesh, which can impact performance. To fix this, after completing your edits to the Sidewalk Blueprint, press "Bake Cap To Static Mesh" to convert the Dynamic Mesh into a Static Mesh. If you modify the shape of the sidewalk, the cap needs to be rebaked to match the updated shape.

Clear Cap Static Mesh - It will unlink the current Cap Static Mesh and will be generated as a Dynamic Mesh.