This is difficult to explain, but I will try my best. This bug only occurs when using "Fabulous!" graphics.

The world border texture is completely transparent in some areas, and only partially transparent in other areas (the light blue transparent diagonal lines). When standing in a location such that two instances of the world border are rendered overlapping each other (such as standing outside the world border near a corner), the world border positioned further from the player will still render even if obstructed by an opaque block when viewed through the partially transparent areas (line blue lines) of the nearer world border. 

This bug only occurs when outside the world border in the positive Z and negative Z directions. It does not occur when viewed from outside the world border in the negative X and positive X directions.

Steps to reproduce:

1. Teleport to the corner of the world border with /tp 29999984 ~ 29999984
   2. Place stone blocks in the corner
   3. Go outside the world border (e.g. using an ender pearl, teleporting, etc.) in the positive Z direction
   4. Observe the visual bug

The first image is a screenshot of viewing the world border from outside in the positive Z direction with "Fabulous!" graphics. The second image is the same as the first image but using "Fancy" graphics. The third image is using "Fabulous!" graphics but viewing from outside the border in the positive X direction.