Using the world border command in the end, regardless of what you type, will not change. There is a random world borderline that shows at -6000 on the x axis but not at the positive X or Z minus or positive coordinates, no matter what size you type in the command, the world border will not center 0,0 or change size.

The other 3 sides of the border have invisible walls.

The second and third pictures will show me changing the size from 100 to 1000 and nothing happens, and it is supposed to be centered around 0,0 coordinates.

This is a vanilla survival multiplayer.