Any datapack changes or additions to the worldgen/flat_level_generator_preset registry are not reflected in the Customize menu for Superflat world type.

Attached is a datapack that adds a new superflat preset and tags it to be visible, as well as edits the Redstone Ready preset to change the icon and make it generate features and lakes. Adding this datapack and opening the superflat presets menu does not show the new preset, nor does it show the edited Redstone Ready preset. Selecting the Redstone Ready preset and creating the world, it will not contain features or lakes as defined in the datapack.

The datapack is made correctly to the best of my knowledge, it validates properly, and using external tools to force its contents to always be loaded the changes are reflected and do function; this leads me to believe the issue is the contents of the datapack not being loaded until a point after the presets screen is accessible.