The world border doesn't appear + you can walk past where it is supposed to be, but can't break blocks there and lose HP.
I'm guessing it's caused by the world border in the overworld being 2500 blocks wide, and it transfers to the Nether when it is not supposed to.
- duplicates
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MC-121997 Every dimension's world border is operating independently, and doesn't appear where it actually is
- Resolved