Unless you cycle through all world types and get back to the Default world type, a datapack's modifications to the Default world type will be ignored.

To Reproduce

1. Download the attached datapack. It overrides the Default world preset so that the Deep Dark is the only biome that generates in the Overworld.
2. Without cycling through the world types, create a new world with the datapack enabled. Observe that the settings in the world preset file are ignored.
3. After cycling through all of the world types until you get back to the Default world type, create a second new world with the datapack enabled. Observe that the settings in the world preset file are respected.

Additional Info

Upon further research, it appears that this bug was temporarily fixed in 23w07a. It was then reintroduced in 1.19.4 pre-release 1.