The bug

Every dimension's world border is operating independently. Changing the world border in one dimension doesn't effect the other dimensions, and in the case of the Nether and End, won't visually change location even though it has.

How to reproduce

1. Go into creative mode and do this command in the overworld:

   /worldborder set 1000
   

2. Go to the Nether and /tp to the world border there:

   /tp @s 0 50 500
   

3. Do this command while in the Nether:

   /worldborder set 10
   

   It looks as though the world border hasn't moved at all.

   /worldborder get
   

   and see that it says the border is 10 blocks, even though you can see otherwise. It looks like the original 1000-block length border.

4. Stay within the bounds of the world border you see and put yourself in survival. You will die. The world border you saw wasn't the world border, it really did move to 10 blocks wide.
5. Go back into creative mode and re-enter the nether, this time teleport to the world border's centre.

   /tp @s 0 50 0
   

6. Go into survival and walk around. The actual world border is invisible, but it's still at 10 blocks wide. You'll experience an invisible force hindering your walk.
7. Return to the overworld and do the /worldborder get command again. Now it tells you it's 1000 blocks, which it is... for that dimension.
8. Repeat for the End, but use a number other than 10 to ensure that the Nether world border and the End world border are operating separately from each other as well as the overworld's.
   If you do the /worldborder get command in all three dimensions, you will get three different answers... and the Nether and End's world borders will not be where they look like they are. They will look like they have the same border as the overworld.
9. Repeat the process, changing world border centres instead of size. Changing the worldborder's centre in the Nether won't be the coordinates you put in, they'll be divided by 8. All three dimensions can have different world border centres as they are all operating independently.