The worldborder isn't fixed on a position. It slides around if you move and it's really difficult to see where it is. You can also see it trough blocks.
How to reproduce?
If you load a new world (or any existing world) and teleport to the worldborder with
/tp 29999984 ~ ~and if you move around you can see the worldborder sliding. Also if you set the worldborder to a custom size, the same visual glitch happens.
This is the seed of the world where I took the screenshots. (The worldborder was set to 250)
Seed: 8031778880841885758
Duplicates MC-163144.