Steps to Reproduce:

1 - Create a new world and note the surroundings.

2 - Exit and go back to the saves menu and click Recreate.

3 - Click on "More World Options" and remove the seed for the world generator.

4 - Click "Create New World" and the terrain will be the same.

Observed Result: The world is exactly the same as the previous one.

Expected Result: A new seed would be generated.

Note: I suspect this bug has to do with this fix -

1. MC-187366

I think it's not property clearing the world seed when the textbox is blank. However, entering a new seed manually works just fine, so this is a work-around.