If you open a furnace, blast furnace, or smoker’s GUI while it is smelting an item, and then re-open the GUI whenever it’s done, the arrow in the furnace will stay as if there is an item partially smelted still in the furnace. For example, if I smelt a stack of iron ore, and I leave the GUI as the first iron ore is three-quarters smelted (therefore making the arrow 3/4 full), and then I return later when everything is done smelting, the arrow will still be 3/4 full, just like when I left the furnace earlier. Has no bearing on the actual game, the arrow will reset to normal once you smelt something else, however it is relatively annoying to always have the filled up arrows.