If you set the worldborder to a number lower than 6, it appears on the center of the block
To reproduce:
Set the worlborder to 5
MC-53574
Worldborder with less than 6 blocks appears wrongly placed
Resolved
Works As Intended
0
This is due to your center. If you centered it on a coordinate like 0,0 then it automatically adjusts to 0.5, 0.5. Use decimal format such as 0.0 0.0 to place it exactly where you want.