To reproduce:

1. Create brand new world in 1.16 Pre-release 2 with any settings
2. Save and Quit to Title
3. From main menu, go to "Single Player"
4. Select world to recreate
5. Click "Re-Create"
6. Click "More World Options..."

"World Type" is stuck on "Custom" regardless of the original world type for the save gave that was created. Note that the world type from the original game is in fact the same world type for the re-created world, but the option to change it is not available.

In 1.15, this was working; a player could change the world type or any other option in the re-create screen.