All FurnaceRecipe experience values, including custom recipes with an experience value defined, are set to 0f once registered. It looks like these values are ignored upon FurnaceExtractEvent and instead vanilla logic is used to calculate experience.

Bukkit.recipeIterator().forEachRemaining(recipe -> {
  if (recipe instanceof FurnaceRecipe) {
    System.out.println(((FurnaceRecipe) recipe).getExperience()); // always 0.0
  }
});