The "Auto Recipe Discovery" config option is supposed to automatically copy furnace recipes that create food to be usable on campfires.
However, when furnace recipes are changed (for example with CraftTweaker), the copied furnace recipes aren't updated to match. They will be updated if you do /campfirebackport reload or reload the config, though.

Example:

1. Make sure the "Auto Recipe Discovery" config option is set to anything other than "neither".
2. Add a furnace recipe with CraftTweaker that creates a food item, such as: furnace.addRecipe(<minecraft:mushroom_stew>, <minecraft:planks:0>);
3. Enter a singleplayer world or run /mt reload.
   -> The furnace recipe was added, but not for campfires.
4. Run /campfirebackport reload.
   -> Now the recipe was added for campfires.

First reported by KibaTheBarbarian here.