Thanks for your interest in my mods. It's not a bug, exactly. The arrows are on the sides where you can input and output ingredient fluids (or results). There aren't any vanilla recipes that take fluid ingredients in a furnace, but they do exist in mods. The game does not appear to display arrows for the fuel fluid inputs. So that's what you're seeing. If you have any recipes from mods that are in the "smelting" crafting category and use fluid inputs/outputs, then the arrows are useful, and only then. Nothing I'm aware of I can do to change it, unfortunately.
