A re-created "Default" world has world type of "Custom"

When re-creating a world that was created in the 21w06a snapshot, the world type cannot be changed. It is set to "Custom" and greyed out, regardless of the actual world type.

To reproduce

1. Create a new world with the "Default" world type.
2. Use the "Re-create" function to copy this world.
   → The world type is greyed out and set to "Custom".