Top left (Previous Page) button turn into clickable button in first page when you click on "Defaults" button
Go to Singleplayer > Create New World > More World Options... > World Type: Customized > Customize
Now, click on "Randomize" then "Defaults", warning pop-up will appear, if you click on "yes" or "no" button, "Previous Page" button will be clickable
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However, this issue is a Duplicate of MC-77608.
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