Singleplayer worlds can turn on cheats even if they are disabled while creating the world. This is done by opening the world to LAN and enabling cheats. This also works for hardcore worlds.
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ampolive
Duplicate of MC-134, resolved as "Works As Intended".
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We're tracking this issue in MC-134, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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Pretty sure this is an intentional feature and not a bug