You are not able to modify the world border in the Nether, End, and custom dimensions.
/worldborder center 0 0
/worldborder set 50 1Environment
Windows 10, Java 8.0.2510.8
MC-190252
Can't change world border in other dimensions
Resolved
Duplicate
0
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We're tracking this issue in MC-121997, so this ticket is being resolved and linked as a duplicate.
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