When using the /fill command to place walls, the topmost changing layer doesn't properly place; the block will not exist until a block update happens near it, but still prevents other blocks from being placed in that location.

Repeating the command makes further layers of invisible, non-solid wall blocks at the next level down from the first layer, and so on until all wall blocks in the affected area are invisible/non-solid. Also, it seems to treat the visible walls as a separate block, as the visible walls are replaced by a command to place them, but the invisible ones are marked as not needing to be replaced.