Greetings again, Reika!
Thank you for your patience with waiting for a response regarding the recipe. After thoroughly investigating the matter, the development team deemed the Horn Glue a proper part of the recipe. Since this seems to be more of a suggestion to change this, I have made sure to move this thread from "In-Game Bugs" to " Items, Synthesis, and Gathering," so that you may further discuss this. Thanks again for your patience!