When in game, at the world border while it is shrinking , you can't modify blocks where both x and z intersect on the world border if they are both negative.
Steps to Reproduce:
1. /worldborder set 59999967
2. /tp 29999980 ~ 29999980
3. Break/place a block where both world borders intersect
4. /tp -29999980 ~ -29999980
5. Break block where both world borders intersect
Observed Results:
The blocks with x and z=29999983 will be breakable, but the blocks with x and z=-29999983 won't be breakable.
Expected Results:
I could break both corner blocks.
Videos:
At the north/west border, trying to place a block at the corner. Only able to once going outside of the border due to MC-223613
[media]
At the south/east border, able to place and break blocks
Linked issues
is duplicated by 2
relates to 2
Attachments
Comments 13

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I can't get a video, but to reproduce.
1. Set world border to 59999999 in a large time period
2. Try destroying blocks around the world border on each axis.
3. 1 axis should let you destroy the blocks that the world border is on, the other won't.

Steps to Reproduce
1. Use a command to set the world border to less than the original length Ex:/worldborder set 59999999 1000000
2. Teleport to world border on x axisEx: /tp 29999980 ~ 0
3. Break the block's with x=29999999.
4. Teleport to world border on z axis Ex: /tp 0 ~ 29999980
5. Break block's with z=29999999
Observed Results
The blocks with x=29999999 will be breakable, but the blocks with z=29999999 won't be breakable.
Expected Results: Either both axis's with their variables equal to 29999999 should be breakable or they shouldn't be breakable.
The value 59999999
provided is invalid syntax. Please provide practical commands that function, and are not examples.

I'm testing them, I said 59999983 not 59999999, you didn't look at it correctly.
The screenshot in @unknown's comment clearly shows that 59999983 cannot be used in the command, so I'm not sure what you're talking about.

59999967 sorry

Can confirm in 1.21.1
Since you are the creator/reporter of this issue @unknown, you can add affected versions yourself. Letting you know in case you were not aware. 👍
Can you please provide a video showcasing the bug?