The center coordinates of the world border are not divided by 8 inside the Nether, like they should. So there is an offset between Overworld and Nether world border.

Steps to reproduce:

Create a creative world.

/tp 8000 ~0 8000
/worldborder set 80
/worldborder center 8000 8000

Build a Portal to the Nether and step through.
Inside the Nether, you will be around 1000/1000.
You will be outside the world border!

/tp 8000 130 8000

You will be at the center a very faint 10x10 world border column.
The center should be at 1000/1000 instead.