The Bug :

The world border cannot be different in a custom dimension and in the overworld. If it is modified in one, it is also modified in the other. This bug is due to the fix MC-121997, which solved a bug on the worldborder which could, at that time, work independently but not correctly. The MC-121997 fix seems to have simply removed this feature, instead of fixing it.

This bug appeared in 1.18-pre1, at the time of the fix. (verified)

This bug is now very annoying for map creators, but especially for datapack creators, since creating a custom dimension to use the worldborder is the only way for us to measure real time without lag.

How to reproduce :

1. Create a new world and a custom dimension using a datapack. The worldborder is 59999968.

2. Modify the worldborder of the custom dimension while being in the overworld with this command :

execute in custom:world run worldborder set 10

3. You can see that the world border has changed in the overworld and in the custom dimension, while before the MC-121997 fix, it would have just changed in the custom dimension.

Observed behavior :

The world border has been set to 10 in the overworld and in the custom dimension.

Expected behavior :

The world border should be set to 10 in the custom dimension and stay at 59999968 in the overworld.