World border textures follow the camera when the player is moving. That isn't how it behaves in previous versions.
Steps:
Go to 30000000,30000000
Go next to the border
Move around, and the texture will follow you.
MC-213426
World border texture follows the camera instead of being static
Resolved
Duplicate
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This appears to be a duplicate of MC-206805, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.