where are you calling addAllRecipes? because you'll probably want to call that in your mod's post init method so other mods can register their recipes before you add them. Or, a better approach would be to access the FurnaceRecipes from the getFurnaceResult. get the result from FurnaceRecipes, double the stack size, then return it. use the exact code from the for loop in your addAllRecipes method, but instead of getting the recipe from the list, call
FurnaceRecipes.smelting().getSmeltingResult(itemStack)