I had a world locked on hard difficulty, but when I was in the Nether and checked it, the padlock was unlocked and the difficulty could be changed. It also unlocks when traveling back from the Nether. Saving the world with locked difficulty does not fix the issue.
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I did attempt a search and found nothing.
Duplicate of MC-44471 – If you have not, please use the search function in the future, to see if your bug has already been submitted.