When you plan a structure to a building, you can use the tile borders on the inside to create extra walls pre-finalization.
When you finalize the plans to the structure, the additional interior walls will vanish. However, it thinks that there are still walls there.
This is a photo of the empty tile border with the correct tool equipped.
"Add to crafting window" will tell you there is already a wall there and will not allow you to use Crafting window.
The only way to get the interior house plans that were made before finalization to appear is to either,
- Wait until building is finalized before placing them.
-relogging so the visual error can be reset.