Trivial bug.
Steps to reproduce:
Open minecraft
Click "Singleplayer".
Click "Create New World"
Change game mode to "Hardcore"
Click "More World Options..."
Notice the two buttons "Allow Cheats" and "Bonus Chest" are disabled
Change world type to "Superflat" or "Buffet"
Click "Customize"
Click "Cancel"
Now the buttons from step 6 are enabled, yet still cannot be changed.
Thank you for your report!
We're actually already tracking this issue in MC-10209, so I resolved and linked this ticket as a duplicate.
However, that ticket has already been resolved as Fixed. The fix will arrive in the next version or is already included in the latest development version of the game.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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