If the block is covered by world border, you can't place blocks directly on them by right-clicking even though you can touch the block.

If the distance between the block behind the border and border itself is zero, you are supposed to be able to place directly on it and the placed block would be inside the perimeter.

If you look at the picture, the leaves behind the border are touching the border and if you could place, the log would be inside the border. But you can't.

This is only a small inconvenience. You had to place the blocks on floor and pillar it up to start working from that block.