Blocks can be placed outside of the world border even though the outside of the world border doesn't allow placing blocks or breaking blocks.

How to reproduce
Start by entering the following command to teleport to the outside of the world border

1. /tp 29999999 256 29999999

1. Proceed to place 1 block inside the world border and you will realize that you can place 1 more block outside the world border (where you currently are) to connect the block inside the world border, try to place blocks without placing a block inside the world border and you will notice that you can't since you're outside the world border.

Observations: Can place blocks outside world border

Expectations: You can't do that since the outside of the world border doesn't allow blocks to be broken or placed.